If you look at class schedules and other school propaganda, you might think that a) students' entire day is filled with hour after hour of rigorous work, and b) even more is needed. But if you followed one student as an anthropologist might and actually kept track of time spent with instruction and practice assignments, the real number is a little less than three hours.
For most students, three hours of formal work is the most they can absorb anyway. This is an upper limit. And if you want students to remember and learn much more than a typical school student, you may decide on even less formal time.