During my undergrad program, a professor told us that he estimated about 3 hrs of study for each hour of lecture, and that probably included some homework. For the upper level classes, some homework problems would take 1 or more hrs to solve and do all the work, which might include programming.
So if one took 16 hrs of classes, one might spend about 48 hrs of studying outside of class. On top of this, one might be doing research which could take 10-20 hrs/wk, and grad students might be teaching in addition to taking courses, doing their own homework, and working on a thesis or dissertation.
3 hrs/day is on the light side.
And certainly some students are bright and absorb the lecture material easily and quickly do the HW assignments. In one of my graduate math classes, one guy would should up with just the textbook, and he didn't take notes - he just listened. He received top marks on HW and tests.