Depends on the school, most people here studied during a rotation or on weekends before Step II. I took about 6 days during my month off while interviewing to study. Typically, after all your 3rd year rotations, you're fairly ready to take the exam. Just brush up on things you don't remember too well. If you're lucky enough to be at a school that allows multiple months off for 4th year, then I would probably take a week to 10 days and no more. Good luck.
