Writing a book, full time, takes between two and ten years. The long poem, John Berryman said, takes between five and ten years. Thomas Mann was a prodigy of production. Working full time, he wrote a page a day. That is 365 pages a year, for he did write every day--a good-sized book a year. At a page a day, he was one of the most prolific writers who ever lived. Flaubert wrote steadily, with only the usual, appalling strains. For twenty-five years he finished a big book every five to seven years. My guess is that full-time writers average a book every five years: seventy-three usable pages a year, or a usable fifth of a page a day. The years that biographers and other nonfiction writers spend amassing and mastering materials are well matched by the years novelists and short-story writers spend fabricating solid worlds that answer to immaterial truths. On plenty of days the writer can write three or four pages, and on plenty of other days he concludes he must throw them away.

Annie Dillard, The Writing Life, p. 14