It really depends on what you're studying as well as the material covered on the test you're studying for. If you're taking a cumulative final exam, for example, you'll do best by reviewing for 10-15 minutes every day with an additional 20-30 minute study session each week to synthesize everything you've learned. For a smaller test that only covers a limited amount of material, you might do fine with 2-3 20-minute sessions the week of the test. Taking a practice test early on can help you hone in on how well you grasp the material and what you need to work on so you can plan appropriate study sessions.