Balance is, in my opinion, the key to success. I'm a 4.0 physics/math student at a well-recognized school, and study about 5-10 hours a week.
The human body is not a machine for studying, but one with many complicated needs and desires. I have found that when I study instead of exercising, sleeping well, eating well. spending time with friends and family and doing mundane activities once in a while, my academic efficiency rapidly diminishes. When I feel well and motivated, I have a much easier time acquiring information and understanding concepts. My concentration and creativity for problem solving also seem to be much better.
So in general, I would say it's better to maximize the efficiency of your study time rather than cramming in more than 20 hours a week. A few hours where you are really concentrated and feel the motivation to learn can be much better than forcing yourself into long study sessions and missing on other important activities for your person.