For too long, learning at school has been tied (figuratively and literally) to the old bell schedule and the Carnegie unit. 50 minutes to one hour per subject per day. Oh, then we got really creative and came up with the block schedule: 90 minutes to 2 hours per subject on alternate days.
Slowly, we are realizing that learning and time do not have to be conjoined. It is not only possible, but possibly much better for students to learn at varying paces, based on the subject matter, availability of resources, their particular learning strengths, interests, and weaknesses--moving toward common goals, but arriving from different directions.