Are you asking about just maintenance service, or are your techs expected to paint as part of the turn? What's the average age/condition of the units?
Because there are a lot of variables in turn times, if you're concerned he's padding hours, your best bet is to pop in randomly and unannounced during the turn. Or have someone work side-by-side with him for a couple and see what the turn time looks like. Have you spoken with him about your concerns regarding turn time? What does he say about the hours? I'm a big fan of addressing the issue at the source - not only would I appreciate the same courtesy from a supervisor, it will also put him on notice that there is an issue and you may see improvement as a result.
If paint is not part of the turn process, I wouldn't expect more than 4 (for an easy turn) to 8 hours (for a more involved unit). But that's assuming that you have a good preventative maintenance program in place, and you're not looking at a lot of extra HVAC/appliance/drywall work. If you're talking about something that leans more towards "rehab" than "turn"... it can go up from there.