Writing a book can be compared to a pregnancy, but unlike a pregnancy, there is zero guarantee about how long a book will take you to write.
First of all, there are many different kinds of books. Non-fiction books require a lot of research. Fact checking is an inevitable part of the book creation process, and it can take a while.
That said, sometimes fiction requires even more fact checking than a non-fiction book. Readers demand that the story make sense. I’ve known people who have developed worlds of their own. Then they don't write a story within the framework they spent years perfecting.
I wrote the first draft of The Corsican in about 30 days. Rough drafts, however, are known to be terrible. Everything gets fixed in edits, and it took a long time to edit the book. Despite taking a long time, it didn’t take as long as it should have. How you perceive a book is finished matters a lot in getting the finished product down.
Bento Box was also a Nanowrimo project, and I raced it. It was cleaner copy than The Corsican, thanks to the learning curve. There was still a lot of rough edges, but it took less time to clean up. I sought professional help. She only took a week to go through my work and point out all its shortcomings. It took me four months to incorporate the changes she suggested.
I had a full-time job while I worked on Bento, so I was only able to work on it in the evenings and on weekends. It would have been faster had I been able to focus my whole attention on it daily. So few people have that luxury, and I wasn't one of them.
This isn’t even addressing the problems that real life will throw at you. A dying pet or a child’s problems with school can take away from anyone's best intentions. Or the month you’ll end up having to stop writing because everyone comes down with the Mega-Flu. Or the week your parents come to visit.
Writing a book is a project based business, and as you continue to write, you become more confident about what you can pull off, and your process gets faster. Better is something that steadily unearths itself, like erosion. You’ll also get better at telling how long it will take you to write a book, because you’ll get practice at it.
Honestly, it doesn’t matter if it takes you years to finish your book. If you finish it, you’re already among the select few who have managed to get that far. You don’t have anyone to answer to but yourself, after all. Write the story you want to tell, no matter how long it takes.