It’s a rare occasion when the words just flow without interruption. More often they flow for thirty seconds, five or ten minutes at a time. Then you stop and think, stare at the page or the screen, look out the window, or whatever it is that you do, and then write for another thirty seconds, five or ten minutes. Once in a while, you’ll catch fire and in a white heat type as fast as you can for an hour or two, even three or four. That has happened to me — maybe ten or fifteen times over the past twenty-five years. For most writers, nearly every day it’s a matter of hills and valleys, with pauses in between.

—Jerrold Mundis, Break Writer’s Block Now!

Another bit from the same book that particularly struck me:

The average full-time writer puts in four to four-and-a-half hours a day, five days a week; the average part-time writer puts in one to two hours a day, five or six days a week. . . . Writing is energy-intensive. Overreaching invites burnout and block.