Taking practice tests are a good tool for improving, but only when you analyze it to identify your strengths and weaknesses. Each practice test has several topics - Airway, Cardiology, Pharmacology, OB, etc. - that the questions fall under. When you're grading your test, figure out which topic the question falls under and whether or not you got it right or wrong. After you're done grading your test, you should know how many airway questions you got right and wrong, how many cardiology questions you got right and wrong, and so on. You should be able to determine where you need to focus the lion's share of your studying after that.
In college, I would usually study for about 50 minutes with a 10-minute break for about 2-3 hours a night. Each person has their own studying habits, though...find what works best for you and use it.