The time you spend on task has some relationship to the quality of work you end up producing. A good gauge to follow is to perform 2-3 hours of schoolwork outside class for every hour of class time. Yes, this means for a full-time student with 15-hours of class per week load the recommendation is to do between 30 and 45 hours of homework each week.
Sure, that's a big jump, especially if you breezed through high school or previous years of college on less. This estimate simply reflects the time it actually takes to learn effectively. It's not steadfast and set in stone. If you find yourself really grasping the concepts of a chapter after a half-hour, feel free to stop. The key here is to set aside this time exclusively for studying. If you get done earlier than expected - that's a bonus!