You don't have to deal with towers much, do ya?
At PDK, they scheduled three hour slots with the hope of having a solid 1.0 to 1.5 once outside of the Class Bravo shelf. We were surrounded by five Class Deltas to get past on the way out except to the north. Then, we'd sometimes have to battle with the bizjets heading in.
Here, we schedule in 2.5 hour slots. At Austin, it's just busy enough we spend at least twenty minutes getting off the ground and taxing back in. Sometimes, it's worse. The average logged time is 1.7 hours with about 1.4 in flight training. Today's student was 1.8 with nearly .5 on the ground getting stuck behind two airliners and a King Air on departure. Taxiing in on return took no time at all.
So, I tend to think of an average lesson around 1.8 hours. Hence, my once a week estimation. BUT, if he's out of one those quiet little airports with little going on, it might actually be just an hour logged and a very fulfilling lesson. I'm not sure what that's like. I've not experienced it since I did instrument training at KGVL.