Imagine you are sipping tea or coffee while discussing various issues with a broad and diverse network of students, colleagues, and friends brought together by the common bond of physics, graduate school, and the physics GRE.
Depends on how you define study. If you restrict it only to reviewing stuff for an exam, then I don't think I've really gone over an hour for that. However, on the other hand I have spent easily 10 hours straight (or more) on a single homework set from a particularly difficult professor, though the problems usually were pretty interesting so it didn't always feel this long. I'm not totally certain on the time as a result.