In my particular shop, we have a punch clock, so the hours are definitely documented. They average seven and a half to eight hours a day. Two days a week they're allowed to work late because the staff work late. If they do stay after a certain hour, we notify security that we're keeping inmates late. It's only if they want to. We don't have any who refuse those extra hours.
Some other shops, such as janitorial work or cleaning jobs and kitchen jobs, probably have half that many hours. For some places they just touch the doorknob, show up, say they're present, and go back to their cell and read a book or sleep.