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 ever 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.
Octavio Paz cites the example of "Saint-Pol-Roux, who used to hang the inscription 'The poet is working' from his door while he slept."
Annie Dillard, The Writing Life
Ah, Annie, you entertain and inspire me. I suppose the biggest thing with writing is that you have to keep at it.
Haruki Murakami has recently published a big novel (called IQ84) of 1000 pages in three volumes, in two books. It took him three years - or about a page a day. He writes every morning and in the afternoon trains for marathons.
Writing a novel is another kind of marathon. Murakami says you need mental and physical strength to write (and the effect of exercise on the mind has been documented).
I'm writing every day now, but I need to exercise more.