This is some sharing from me, on how I study, may be as a guidance to those who are looking for ways. There are many ways, and I think most importantly, it must suit yourself.
Personally, I like to pre-read what is going to be taught in the week, on the weekend, before the class is taught. So, that is round 1 reading. It could be 10-15 minutes per subject, just glancing through, to give you a better idea. (an hour or 2 per week on weekend)
Then round 2, will be on the night before class, or day of class, I would spend say 10 minutes each subject, to read through it. Know where you understand and where you don't understand. Make some marks about it and concentrate in class or ask in class in particular about those stuff. (I think you need to be consistent in doing this and not because I have exams tomorrow in that subject, so i am not doing it today...and you can keep on putting off). (not more than an hour a day)
Round 3, pay full attention in class and interact in class. Ask questions etc.
Round 4, on the same day after class, even in between class, read through what is taught. Jot down where you don't understand etc. (15-30 minutes per subject or so, an hour or two per day)
Round 5, get help from the teachers or friends on stuff you haven't understand.
Round 6, try your homework/assignments when your memory is still fresh. Try it yourself first and then yuo can discuss with your friends.
Round 7, revise it over the weekend. (say half an hour a subject per week).
Guess with these 7 rounds of preparations, you will be ready for exams. The time is just a guidance to show that you don't need whole lot of time to do it.
All those except homework could be done within 20 hours a week.