Depends how hard you've been driving the car.
Without a timer and if you've taken it easy off boost mainly for the last 5-10mins then leave it idling for a couple of mins before switching off.
If you boost it to your doorstep then could be anything upto 10mins, 30mins dependant on circumstances..but generally a few mins is sufficient.

ALWAYS let the car idle after being driven tho. Even if it's only for 1 minute.
It's not to let it cool down as such (altho that is part of it), it's to allow the oil to circulate and prevent bearing seizure at high temps.