Hmm theres not a set amount of hours because everyone is different, some kid that soaks up information from the get go is going to have a much easier time then someone that has trouble retaining information that has to put 5x the amount of hours to reach his level.
I think the general consensus is if you feel confident in your knowledge and the grades reflect it.
If you can and have the determination, then studying a lot can only benefit you as long as you don't get burnt out.
Im doing art design for example and have classes 3 days of the week, yet apart from lazy Sunday, I put 8+ hours in my craft and im the best in the class, I do it simply because I have a passion for it and I have big goals in working in a well known studios...