I think about 6 would be my upper limit, haven't done that yet but would like to! We book out 1.5 hour slots for even a 0.7hr circuit lesson.
Would depend greatly on how much homework the student has done, and the weather - dodging clouds, talking them through landing after landing with heat-induced up- and downdraughts on short finals, over the driving range (why is there a golf course near every airport?) tends to raise the stress levels a wee bit.
Also, doing most of the day in the C152 and then changing to the Arrow or other breed for the last flight tends to make that one a bit safer if you realise just how great the accident potential is and compensate (or overcompensate) in safety terms. If you don't though, well, good luck with that last flight!
Safe flying
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Confident, cocky, lazy, dead.