Most of the learning, other than the motoric skills, are learnt on the ground, not in the cockpit. You are responsible for your students progress, and if the flight cannot be done, including a good pre- and postflight briefing within the allotted time, it should be rescheduled. Either allot more block time, or schedule more time for ground sessions. Personally, I don't fly if I don't have about 1 hr between flights, depening on the situation. This does not include long briefings.