Depends what year you are talking about. If you're talking about 1st two years, if you stick to a good schedule of working out, going to class, and studying then excercising is not too difficult.
3rd and 4th years it may be easier for some and harder for some depending on the rotation that you are currently on. If you are on a hard surgery rotation then you're going to have little time, but if you're on an easy psych rotation then you'll have plenty of time. So far in my third year, I've had a lot of time to exercise.