Depends on the person, subject, course load, and the amount of time you have. Personally for all of my tests, exams, etc. I always use the 30.5 rule. 30 minutes studying, 5 minutes break, repeat. I do that for about 4-5 hours on Mondays, Tuesdays, Wednesdays, and Thursdays. Fridays are my relaxing day – 10 minute flash card review, 5 minute break, repeat for an hour, then read a summary of your notes.