A possible Schedule could be
7am show up, dive site briefing and prepare gear
8am in water 30' for 60 minutes
9am out of water discuss dive, change tanks brief next dive
10am in water dive 2
11 am out of water discuss dive, secure gear
Break till 300 pm
3pm brief next dives and set up
330 pm back in the water for dive 3
430 out of water discuss dive brief next dive and switch gear
530 pm back in for last dive
630 out and de brief, break gear down.
Now this is just one possible scenrio and would give a student 4 hours of bottom time. Many other variables come into play like class size, enviroment, student preparedness, ect.
.
I'm thinking if I limited my classes to students between the ages of 17 and 35 or so, two thirds to three quarters of them might make it through that schedule in decent shape. I don't actully run into that many experienced certified divers that are overly happy doing 4 hours of bottom time in our 78 (+/-) degree water.... 2-3 hours a day is quite enough for a significant percentage of pretty active divers.