How long does it take to write a book?

Writing a book requires you to put a lot of effort. I once tried to write a book and I realized how hard it could be to author a book. You need to fact check each and every thing. You need to make sure that your grammar is flawless. You need to maintain a good time. This process is quite time-consuming. As a result, I reckon that even writing a small booklet could be very time-consuming. However, this depends on how lengthy a book actually is. So, how long does it take to write a book?