Say a book is 60,000 words. You should be able to average 500-1000 words a day. So that's 60-120 days, or four months, writing one to two hours a day.
Why doesn't this happen?
1) People don't write every day, or even four or five times a week. They spend whole months without doing anything on their projects.
2) What about research time? It's possible that the writer doesn't know enough about the subject to write a book yet. I'm assuming here that this book is about something that you know something about, or could research in about a year. So, if you want, give yourself a year and four months, to research and write the book.
Research time is not trivial, of course, but you can't even talk about writing a book unless you know enough about a subject-matter to conceive of the book project. If you imagine spending most of your research time for a year on a single subject, you will know enough to write a book by the end of the year.
A few caveats here. I am also assuming that, by reading and researching, you will come up with worthwhile ideas. I don't know how to tell you how to do that. When I read, almost anything at all, I naturally generate ideas, because I have an active intellectual engagement with the subject-matter. I've learned that you can't take that for granted.
Secondly, many people seem to think that you can simply make the pages pile up without sitting down to write them, as in some Henry James story. Many people will complain that they cannot write every day, and hence my plan is unrealistic. Yes, it is unrealistic if you can't do this.
Thirdly, you might not want to write a book every two years. I certainly haven't done so in my career. I have had time to write more, certainly, but I think you need to think about your contribution and what that entails. I could have written more, but with more repetition of ideas and patterns of thought.