Remember how much you studied in college? For some, the answer will be "Quite a bit," while for others, the answer will be, "Are you kidding? I didn't even buy some of the textbooks!" Often, both answers (and all others) are based on how much a student had to study in order to achieve the desired GPA.
In other words, for some students A's were easier to obtain than for others; some students were satisfied with several B's.
Now, the focus is different.
That having been said, the question still remains, "How much should one study?"
The answer is, of course, different for different students ... but not as different as you might suspect. The "rule of thumb" is this: three hours for every class hour. That means, if you are taking 15 credits, study 45 hours.
That turns out to be a 60 hour week. That's about one-third of the hours in a week.
You ought to sleep about one-third of the hours in a week (7 days times 8 hours = 56 hours). That leaves you with about one-third of your week to attend to the rest of your life. Eating, playing, relaxing, socializing, cleaning, commuting, praying, playing the piano, reading for enjoyment, continuing with hour hobby ... whatever you choose to balance your life.
The really important question is "How should one use those 45 hours of outside-of-class study time?" You'll find the answer to that ... from many perspectives ... on these website pages. You can begin by turning to the Law CATS page, where you'll find a brief overview of what ought to be done each week in each subject. From there, you can begin to construct your weekly study plan.