How Long Should it Take to Write a Book?

 Out of a human population on earth of four and a half billion people, perhaps twenty people can write a book in a year. Some people lift cars, too. Some people enter week-long sled races, go over Niagara Falls in a barrel, fly planes through the Arc de Triomphe. There is no call to take human extremes as norms. [Mainly, the time it takes to write a book depends on the book. Erle Stanley Gardner wrote six crime novels a year.] 

Annie Dillard, The Writing Life, 2013