In a college course on John Milton, I believe I remember the professor saying that Milton had an extremely rigorous daily reading schedule, something like one hour of Greek and two hours of Hebrew every morning. Are there sources that indicate what Milton's reading habits were?
We can't really say for sure what his daily reading habits are like (and he almost surely maintained different schedules over the course of his life - certainly blindness must have had a significant impact).
In this case, however, I suspect your professor was referring to Milton's reading of the bible, which he could do in the original Hebrew and Greek. Milton is known to read the bible every morning, a habit he retained even after losing his eyesight:
He was an early rise (sc. at 4 o'clock mané) yea, after he lost his sight,. He had a man to read to him. The first thing he read was the Hebrew bible, and that was at 4 h. mané, 1/2 h. plus. Then he contemplated.
Aubrey, John. "Minutes of the Life of Mr. John Milton." The Early Lives of Milton. Ed. Helen Darbishire. London: Constable 15 (1932).
The same source goes on to say, the reading resumed at 7am and continued (in conjunction with writing) until dinner. Beyond this, however, we don't really have too much much details on his daily habits - except that he must have read extremely extensively, in all likelihood far more than the three hours per day.
We can gleam some sense of Milton's beliefs on how much to read from the curriculum he prescribes for his pupils:
Parker tries to give an idea of the disciplined rhythm of life in [Milton's] house, conjecturing form evidence in Of Education and An Apology: reading form five or six in the morning, until the mind was saturated; some exercise or perhaps music around lunchtime, to set the mind back in receptive frame; more bookwork in the afternoons; a plain supper, and expounding scripture until bedtime.
Brown, Cedric C. John Milton: a literary life. Springer, 1995.
The prime source for Milton's own reading is his Commonplace Book, which Milton maintained for several decades after graduating from Cambridge. A commonplace book is essentially a kind of scrapbook, and Milton filled his with notes and quotes from his readings. This has provided later scholars with some picture of how extensively he read, as well as how many languages he read in.
The readings contained in Milton's Commonplace Book is already formidable, but it isn't a complete documentation of all his readings. Several books he purchased and annotated survives, which don't doesn't appear in the manuscript. Moreover, other readings can be inferred from references that has appeared in his writings.
Given the vast amounts of literature he is known to have consumed, Milton must have read for many hours on most days.