How many hours a day should an undergraduate study?

My university states that an undergraduate student needs to study three hours for every hour spent in lectures, tutorials, labs, etc.

For example, if I have subject which has seven contact hours (3 hours lectures, 2 hours practical, and 2 hours tutorials) in a week, then I need to self-study 21 hours each week.

I know that some students study like 14--16 hours per day but honestly I can not study more than 8 hours per day and I only can concentrate on the first three hours.

Therefore I was wondering, how much do the undergraduates at top universities like MIT or Stanford study? I am aware that the quality of the study is important but not the quantity but thought maybe there is a rough number of hours that one needs to study.