Depends on how much time you're willing to put in.
That's the short (facetious) answer, but it's honestly pretty arbitrary. Depending on the genre, content, and author, two books of the same word count may take a drastically different amount of time to write. To exemplify this, I want to draw your attention to sheer anarchy that being an author entails:
Harper Lee's To Kill a Mockingbird is one of my top three favourite novels. It was published in 1960, when Lee was 34 years old. It was her first novel. In 1961, it received the Pulitzer Prize for fiction. In 1962, it was adapted into a film. Lee died in 2016; it remained the only novel she ever published. (That is, unless you count Go Set a Watchman in 2015, but that whole situation was a mess and it was essentially the cutting room floor of To Kill a Mockingbird.)
Joseph Heller began writing Catch-22 in 1953, and it was first published eight years later, in 1961. He was certainly not a slow writer, however: he would often have figured out the basic plot and characters within one hour of coming up with the novel. If you've ever read Catch-22, you'll know there are a lot of characters.
So many of the greatest works of literature come from authors who only wrote a handful of works over their lifespans, and only began to publish them in their midlife or beyond. On the other hand, some authors function almost indistinguishably from factories, such as Stephen King, who currently has 65 published novels, generally releasing one to three novels every year. Some poets have written so many untitled poems that they are known only by a number, and these numbers can rise into the thousands. John Bradburne, the most prolific poet in the English language, wrote twice as many lines (169,925) as Shakespeare (87,668) and achieved not even half the notability. He also wrote most of those 6,000 poems over a period of 11 years, whereas Shakespeare was active for approximately 30 years. Some of Shakespeare's works were outrageously and unnecessarily lengthy, such as Venus and Adonis, which is openly acknowledged in many literary circles as being desperately in need of an editor. It is 1,194 lines long. For scale, that is just over 1/10th of the length of The Odyssey (12,109 lines). The Odyssey is between ~130,000 and ~160,000 words long, depending on the translation, and can take from nine to over 15 hours to read front-to-back, depending on various factors.
Anthony Burgess' A Clockwork Orange was written in about three weeks as a cash grab. The first Sherlock Holmes novel, A Study in Scarlet by Sir Arthur Conan Doyle, was also written in three weeks. Charles Dickens' A Christmas Carol took six weeks to write, though that's to be expected from Dickens, who was paid a farthing for every word he wrote and was thus highly prolific. The Boy in the Striped Pyjamas was written in two and a half days, which is not remotely surprising. Starkly opposed is The Lord of the Rings trilogy, which took J.R.R Tolkien about 17 years to write, and me 23 years to read past the first page. Ezra Pound's The Cantos took 57 years, and was still unfinished when he died.
Therefore, the long answer is sometime between 60 hours and 57 years.
If you are relatively normal, like me, you're going to fall somewhere in between those two figures. In fact, you very likely will fall somewhere in between six months and three years per novel. I would even wager that the accuracy can be further pinpointed, by taking my very helpful quiz, which is about as accurate as astrology (make of that what you will):
All jokes aside, it's not an easy question to answer. There's some kind of inverse correlation between how long you've already spent on the book, and how long it's going to take to finish. If a book has been worked on for more than five years, it's often because the person working on it falls into a group of writers who are more likely to work on a book for at least ten years. Or so it's said.
Thus, I cannot tell you how long it's actually going to take you. I can only show you how long it's taken me, thanks to the version history feature of Google Docs, and old hard drives.
How long it took to write my first novel, Maximalism:
The timeline is undoubtedly messy for this one. I remember the exact moment the idea spawned, which was after a university lecture on Greek and Roman literature, sometime in early 2020. However, the characters had existed since 2018, of course in a very elementary sense:
These terribly embarrassing drawings from six years ago are a necessary evil in outlining how Maximalism came to be. Yōkō is no longer a (weirdly tender) white guy called Hank, for one thing, nor do either of them have horrendously oversized and deformed noses. During their inception, I intended to write them as high school teachers. Then it sort of just spiraled out of control.

At some point in 2020 I redrew Yōkō closer to my vision, albeit like he was about to appear on an episode of Botched. I am not sure what possessed me to draw his chin like it's used to teach the Pythagorean theorem.
Over time, the composition of his body changed, particularly as I began to write him with spastic diplegia, but also because his utter lack of interest in taking care of himself would not lend to anything close to fitness (AKA, he does not look like what's on the left. At all).
This was also the point wherein I began to work on the first version of Maximalism, originally titled The Maximalist. All of these files are now located on an external hard drive, which were thankfully not encrypted by me during my "I need to encrypt all my files" phase which has caused me nothing but fury over the past five years.
Maximalism took approximately eight rewrites, from early 2020 to late 2023, to be finished.
The oldest completed version I can find is version 3, which clocks in at 153 pages, with a word count of 43,063. I believe the two versions prior to that were closer to short stories. The following files are within the folder for version 3, complete with the content of certain files. These files are almost entirely irrelevant to the completed work, and kind of showcase the sort of redundant neuroticism I had at one point about planning every fine detail.

Version 4 marked an enormous overhaul of the plot, and was completed approximately eight months after version 3. It's 241 pages long, with a word count of 60,797.

Version 5, completed three months after version 4, is 157 pages long, with a word count of 69,416 (I reduced the line spacing).

Version 6 was completed a month after version 5. It is 183 pages long, at 78,274 words. It was the first version to have been beta read, which was by a university professor of mine. She picked it apart brilliantly, and left me with so much new information to work with. It was also not the final version, despite my clear yearning that it would be.

In this folder, versions 7 and 8 are both contained, as well as the file with the comments from my professor (FINAL[84]).
I received the feedback in early 2022, about eight months after finishing the sixth version, and spent the following five months reworking the novel to its seventh version. Version 7 is 208 pages and 94,268 words long.

Version 8 entailed a very brutal editing pass. It dropped to 178 pages, at 78,770 words.
It was completed four months after version 7, a few days out from the new year.
This was the last version I moved to my external hard drive, and the rest remains on Google Drive in the version history. I will call this the final version, being version 9, which is officially 79,300 words and 184 pages long. This version marked the addition of the copyright page, the works cited list, and a few other formatting bits.
I want to make something very clear when we look at the version history: there was a period of almost a year, between December 2022 and October 2023, where I did not touch this novel.

The reason for this is simple: at this point, I absolutely hated my novel. I had re-read those same ~80,000 words well over a hundred times, and experienced a sort of cyclothymia in regard to it. I jumped frequently between perceiving it as a masterpiece, then thinking of it as nothing short of an embarrassment and a complete waste of time. This information shouldn't be sad or worrying, because to me, it was actually fantastic. It forced me into a state of utter neutrality; my own neurosis had pushed me beyond a point of no return, where my ego had no choice but to completely disintegrate. I won't say I became objective, not by a long shot, but I certainly stopped freaking out over whether the book was good or not. Instead, I started to ask myself questions: did I accomplish what I set out to do? Did I write something which I found funny, significant, meaningful, interesting, and satisfactory? When I hit the point where I could answer yes to myself, that's when I decided the book was ready.
Of course, there are still things about that book which don't meet my own standards. There's a lot I'd still change, and I am planning on doing just that sometime in the middle of this year, as one final revision. But I now have an awareness that how I feel about my work will always be superimposed, and I need to know when to stop.
Novels get easier to write as you write them.
Crazy!
There's a novel I'm writing that I don't talk about much, called Late Night. It's intended on being a single novel, with the story divided into two parts. The first part is complete, and is 72,500 words long, at 168 pages. The second half will ideally be about the same length, meaning the finished book will be ~150,000 words long. The majority of the novel was written within a single week in 2021, and I have never edited it, and probably won't once it's done.
The novel was started on July 9th, 2020. That day, I wrote the first seven pages, and proceeded to not write another word until December 21st, 2020. I then worked on it for nine straight hours, then 11 straight hours the following day, with a similar pattern every day until about the end of January. Between December 21 and December 28, I wrote 58 pages. Between December 28th 2020 and January 28th 2021, the page count had jumped to 182, with most of the activity being between the 5th and 12th of January, where I wrote over 100 pages. I proceeded to barely touch it for two years. At some point in that period I decreased the line spacing, meaning it dropped back down to about 140 pages while remaining the same word count, before I picked it back up in January 2023 and wrote around another 28 pages.
My other novel, Beekeeper, had a similar hiatus. It was stuck at around 7,000 words for several years, though I tragically do not have the version history to back that up because the current file is a copy of a copy. I was never at a point where I felt ready to write it, until approximately November of last year, where I attempted NaNoWriMo. I got a lot done, but didn't hit the word count I'd hoped for, falling short by about 20,000 words. I am currently in the process of fighting for my life to have this novel completed by early March, because I am intending on submitting it to a competition.
Beekeeper is more or less the inspiration for my choice in topic this week. It is currently sitting at 40,000 words, and I am hoping to double that in a little under two months. I am completely capable of doing this, and it's not even as strenuous as it appears, being about 750 words a day. I generally write between 750–2,500 words a day, sometimes reaching beyond 3,000 on my best days. Regardless, it still remains a daunting ask of myself, and I'm not going to force myself to hit 80k if the story doesn't actually call for it.
The reason why I don't believe Late Night or Beekeeper are going to require numerous versions and revisions is twofold. Unlike Maximalism, neither of these novels are particularly dense in research. Beekeeper is still somewhat research-demanding, but nowhere near the extent of Maximalism. Late Night requires essentially no research at all, as it is probably the only work where I'll ever follow the advice "write what you know." It is also likely to get me in profound amounts of legal trouble for defamation, which is why I've set it aside for so long until I figure out how to circumvent that.
The other reason is simply experience. Maximalism wasn't the first novel I ever attempted, but it was certainly the first novel I ever completed. There's around fifteen other abandoned novels that came before it, with god-awful titles such as Let's Kill God, Hometown, Untitled document, comedy book.docx, An Open Letter to the Universe, The Eastern Pack, Nature Takes All, Realm Master, Cor Avem, Snuffed Out, In The Valley, and my personal favourite: fuck this.PDF.
Yes, most of those are from when I was about 14.
If you took my quiz, and happened to select graphic novel as the project you're working on, chances are your result was that it would take you five or more years. I'm of course being very tongue-in-cheek about this, but it is also very much true that graphic novels take an extremely long time compared to novels (of course, depending on what you're doing.) That's why I'm not going to talk about String Theory here, because the timeline for that whole thing is invariably like a perpetual trainwreck. Also, I talk about String Theory in like, every other post.
Thanks for tuning in this week! I know that having the opportunity to look under the hood of the novels I was reading like this, especially when I was a younger writer, would've been wicked helpful. I hope it was helpful for you! Please bear in mind that this is not a representation of what writing a novel 'should' look like, and if your process differs, I assure you this is not me trying to change that in any way. (Novel-writing probably shouldn't look like this, truth be told.) It's rather that I think it's important to give writers the ability to look at someone else's process, which we don't often get to see. I like to think of it as the equivalent to a visual artist showing how they sketch out and draft their works.
Note that I may skip a few blog posts over the next two months in order to focus on Beekeeper, but I'll do my best to post at least once a fortnight!