My first novel took probably eight years, start to finish. My second novel took about six months to write. Editing took . . . actually, without checking the dates on my files, I don't know how many years it's been. Two? Maybe three? And it's only just now ready to start querying. I went through eight distinct drafts. But the finished novel is ten thousand times better than the first draft! So, hey, I've learned something.
Bear in mind that I'm not a full-time writer and this was only my second novel ever. And I'm a perfectionist, so I tend to work slowly and methodically, making sure every "I" is dotted and every "T" is crossed.
My third novel is taking a lot longer for the first draft to get written (it's been well over a year since I began), but it has a more complicated plot. It has politics and military strategy, bio warfare, and colonization. Things I know nothing about.
Why, muse? Why must you torment me so?
Whatever works. It does help to know your subject. My first novel took three weeks to write, start to finish, and I had zero time to edit, and had to submit the first draft. It sold as was, so I can't complain, but that was rough.
I believe in getting every sentence as right, as perfect, as possible, and I work very, very hard to make this happen, but I also know that "perfect" is the enemy of "good Enough", and that "perfect" isn't even possible.
So much of this is natural style, natural method, etc. I've always been both very fast and good enough to sell. Some of my best stories and novels are also the fastest I've written. Many of my best sales were true first drafts with no revising, rewriting, or editing at all. Some of my worst, and completely unsalable, are ones that took a relatively long time, and that I slaved over.
There doesn't seem to be any rhyme or reason for it. It's just the way I've always written, and the way it has always worked for me. It's what came naturally when I first sat down to write, and I can't begin to explain it.
But like Isaac Asimov said of his own stories, if I don't get it right after a quick, clean up second draft, no amount of work will ever make it right.