Boice's general advice is far from new, of course. Anthony Trollope worked essentially that way. His maxim was: "A small daily task, if it be really daily, will beat the labors of a spasmodic Hercules." And he did something else, Boice recommends: he kept score. As he tells us in his Autobiography, a page for him consisted of 250 words. And he produced 40 pages a week at his best, 20 pages at his worst. If we assume he wrote 30 pages on average, he must have written roughly 1000 words a day.
Trollope also tells us that "all those I think who have lived as literary men, working daily as literary laborer will agree with me that three hours a day will produce as much as a man ought to write. But then he should have so trained himself that he shall be able to work continuously during those three hours—so have tutored his mind that it shall not be necessary for him to sit nibbling his pen, and gazing at the wall before him, till he shall have found the words with which he wants to express his ideas. It had at this time become my custom ... to require from myself 250 words every quarter of an hour." So his blocks of daily writing must have been one hour long on average (after training himself appropriately).
Trollope did all this before going to work at the post office, which may or may not have been harder than the daily work of a professor.
1. "I found it to be expedient to bind myself by certain self-imposed laws. When I have commenced a new book, I have always prepared a diary, divided into weeks, and carried it for the period which I have allowed myself for the completion of the work. In this, I have entered, day by day, the number of pages that I have written, so that if at any time I have slipped into idleness for a day or two, the record of that idleness has been there, staring me in the face ... I have alloted myself so many pages a week. The average number has been about 40 ... my page has been made to contain 250 words."—This amounts to roughly one double-spaced typed page.
2. See also Trollope on Reading and not Thinking.