Rotation dependent and you just need to be kind of flexible. When I did psych I could go to the gym every day after work. (or before if I was the type to wake up before I absolutely had to) I usually had time to swim for an an hour and do weight training. Really I could have worked out for four hours a day and still had free time for other stuff as well.
On surgery (other end of the spectrum) I stayed active most of the day (walking around the hospital, taking the stairs, utilizing the pull-up bar in the anesthesia supply room between cases) and did my more extensive stuff on my day off. I usually ran 8-10 miles on my day off and only 3-4 on a couple of other days during the week when I had the time. That and just doing some body-weight stuff (push-ups, pull-ups, core stuff) was about all I had time for. The big thing is just finding the motivation to do it when you are tired from working all day. I used it as my time to zone out and turn my brain off. That really helped get me moving when I didn't think I felt like it.
I will say that if you make time for anything make sure you make time for core strengthening. Really helps when you have to stand in the OR for long periods of time in weird positions.