Can I set a time delay before a PM5 turns off with On-Demand Start?

Yes.

When a Power Control Module (PM5-120/240) is set for “On Demand” start, once a thermostat has completed a heating, cooling, or fan cycle, a delay can be set before the relay is turned off. 

The default delay period is 15 minutes, but can be set from 0 to 240 minutes.