Long answer: it's really different for each kind of book. Horizon was a big one but it took me just 6 weeks in total with many days in which I didn't put a single word down.
Sometimes I write a book half way, then I leave it for a few weeks before coming back at a sprint to finish it off ASAP.
This year has been a hectic writing year because I've had to focus on marketing and new softwares for editing books. But for the most part, I've paid more attention to marketing than writing, just to make sure that when I do release my books, I already have an audience that will embrace it.
But a chap a day is usually how I roll. Two chaps if I'm in a big hurry.
Sorry this reply came so late. I should check my author profile more often.