You’ll probably encounter books, courses and seminars that allegedly teach you how to write a book in an absurdly short length of time. Since it’s possible for a book to have just three words in it, it is actually possible to write a book in less than ten seconds!
However, most writers of “real” books take from three months to a year or more to write. And then the book requires more time for revising, editing, designing and marketing.
Very few self-published books come out “on time.” Everything takes longer than you think it will. If you rush, you will make mistakes that will take additional time to correct. It’s much more important to be good than to be fast or first.