Just to get by (not dominate), you need to be putting it at least 40 hours per week in any form (class, reading, studying). How you divide that up is the student's choice. Some don't ever go to class and say it's a total waste.
I always went to class which on average was from 8-12. Usually took an hour for lunch. Then studied until dinner. Studied a bit after dinner (except Fridays) and then to bed by 10. I personally always studied at least 6 hours/day on weekends, and double that if there was a test on Monday.
It was a huge sacrifice of my social life, but I was very competitive and wanted to beat my friends on every single exam. It helped that they worked hard as well and we really pushed each other.