at uic the rotation was the easiest I've had in med school. some work from 8 until 4 but other sections worked from 8 until 11am and then noon lecture and that was it, no other expectations.
at uofc the rotation was quite a bit more involved. I was there from 8am until 5 or 6pm depending on what was going on.
at Ill masonic I'm not sure of the hours but they require a presentation as part of the rotation.
Even though it's fairly easy, if you're applying for rads you'll feel stressed pretty much the whole time as you'll be worried about looking interested without being annoying, asking good questions, getting findings right (as much as you can as a 4th year) and paying attention for hours straight at a time without being directly involved.