This might not be the answer you're looking for, but from my personal experience an average or estimate like that is pretty meaningless. I have seen residents "work" 35 hours per week next to others on the same rotation who will spend 80 hours, much of which is occupied by research or studying while present "at work."
I would recommend not putting too much weight on what the program directors estimate on hours, often they really have no idea. Not to mention, of course, that it will vary trememdously with the type of rotation, eg surg path vs clinical chemistry. Ask the residents themselves how things are, or check scutwork.com to see if anyone has posted work hours (reviews there generally seemed right-on when I compared them to what residents said about a program at the interview).
Finally, as you mentioned, having PAs to help gross can cut the hours down and free up study time, and some programs/rotations do give more work than others, but you really have to talk to the residents who have been through it to know for sure what the hours are and what they are spent on.