It really depends on the person, the course and what your goals are - what kind of marks do you want, what kind of social life, how much money you need etc. (and how easy you find your courses)
Personally I find any more than 2 days work a week (aprox 16 hours) gets pretty difficult if you want to maintain a good balance of uni (class/marks) and social life/not spending the entire semester stressed.
Remember that contact hours are the usually the bare minimum for courses - you will have to do readings/assignments/study etc outside that time.