This will depend on, among other things, the difficulty of your courses and the effectiveness of your methods. Many teachers recommend studying at least 20 min. If your time diary indicates that you presently study less time than that, do not plan to jump immediately to a much higher level. Increase study time from your baseline by seeing weekly goals that will gradually bring you up to the desired level. As an initial schedule, for example, you might set aside an amount of study time for each course that matches class time.
Some classes require nightly work to be successful.
Id those classes and build them into your nightly schedule. Schedule for maximum effectiveness. Tailor your schedule to meet the demands of each course. For the course that emphasizes lecture notes, schedule time for a daily review soon after the class. This will give you a chance to revise your notes and clean up any hard to-decipher shorthand while the material is still fresh in your mind.