I can say this much. IF YOU LOOK FOR IT, you will find that many U.S. colleges and universities actually say somewhere that students are expected to spend 2 to 3 hours in outside preparation as they spend in class. A "normal course load" is four or five subjects, that meet three times, sometimes only twice a week, for an hour. So, a normal load is 10-15 hours, multiply that by 2.5 for homework, and you get a grand total of maybe 35-42 hours.
In other words, something like a normal work week.
MIT, where I went to school, was unusual in listing three numbers for every course: lecture, lab, and outside prep. So, a course listed as "3-0-6" meant an easier course where you were expected to put in 6 hours' homework, and "3-0-9" meant a harder one, where you were expected to put in 12.
I think a "40-hour week" (for both class and homework) is a reasonable ballpark figure. Of course some students put in MUCH less, and some put in MUCH more.
Harvard University, which presumably sets high expectations, says this:
http://www.hcs.harvard.edu/womswim/recruiting/myths.html
"A normal course load is four courses of three one-hour classes per week (12 hours of class each week). Most students spend one and a half to two hours in work outside of class for each hour of class."
That works out to a total of 30 to 36 hours grand total.