Annie Dillard in Literary Hub:
To comfort friends discouraged by their writing pace, you could offer them this:
It takes years to write a book—between two and ten years. Less is so rare as to be statistically insignificant. One American writer has written a dozen major books over six decades. He wrote one of those books, a perfect novel, in three months. He speaks of it, still, with awe, almost whispering. Who wants to offend the spirit that hands out such books?
Faulkner wrote As I Lay Dying in six weeks; he claimed he knocked it off in his spare time from a twelve-hour-a-day job performing manual labor. There are other examples from other continents and centuries, just as albinos, assassins, saints, big people, and little people show up from time to time in large populations. Out of a human population on earth of four and a half billion, perhaps twenty people can write a serious book in a year. Some people lift cars, too. Some people enter week-long sled-dog races, go over Niagara Falls in barrels, fly planes through the Arc de Triomphe. Some people feel no pain in childbirth. Some people eat cars. There is no call to take human extremes as norms.
More here.