I think the number of hours will vary greatly, even within a specialty, depending on practice environment -- private solo vs private group vs academic vs community, etc.
The cardiologists at the academic institution I'm affiliated with seem to work almost as the residents. They round with us, then go to clinic or cath, or whatever they're doing that day, then they come back in the evening to go see patients and do checkout with us. My guess is that unlike us, they don't go home after that; they likely return to their offices on many days to finish paperwork, etc. But, hey, that's just cardiology . . . everyone knows (or assumes) that cardiologists are workhorses anyway.