According to the University of Michigan at Flint's Academic Advising and Career Center, it should take about two to three hours of study time per credit hour outside of class to complete coursework. This is in addition to the time you spend in class. For example, if you have a three-credit hour course, you'll spend two to three hours in class and will also have to study six to nine hours outside of class. Multiply this estimate by the number of classes you're taking to understand how much you'll need to spend each week, and then divide the hours up into daily pieces.
Once you have an estimate of how much you'll need to study each day, you'll need to set a schedule for yourself. Think about how you might study best. Some people, for example, would do well by dividing the total study time into small segments for each of their four classes and doing a little work each day. Others might do better by studying in longer stints and taking a break every other day. Your study time doesn't have to be a long chunk of uninterrupted time either. If you have 30 minutes in between classes, use that time wisely.
Of course, no exact science exists to studying, and you may take more or less time to study for your classes. For example, the Academic Resource Center at the University of Utah says that a class you find easy may require only an hour of time per credit hour outside the class. Something more difficult might take three hours or more per credit hour. Attending review sessions, meeting the professor during office hours, getting help from a tutor and forming a study group with other students can increase the amount of time needed, but it might make the difference between an "A" and a "B."
During your midterms and finals, you'll probably have to allot even more time for studying in order to pass the exams and work on different projects. If you want to help yourself, plan ahead. Start a term paper several weeks before it's due so that you're not stuck writing the entire thing the night before. Take careful notes and rewrite them so that studying is only a matter of reviewing material, rather than trying to learn it for the first time.