A couple of weeks ought to do, although it depends on how well you want to do and how well you did on your shelf exams. If you failed all your shelf exams, would recommend that you study at least two weeks, mabye just doing questions will be enough. If you want to do well because you want to use it when you're applying, then you should study at least a couple of hours a day for a few months. I didn't do great on my shelf exams, studied one week for a couple of hours a day, still did better on Step 2 than I did on any shelf exam.