It's more of a cool-down time for the bearing cartridge. The turbo spools down quickly, not to worry. The concern is shutting down the engine when the bearings in the turbo are still, like, glowing hot. Then the oil will coke up and eventually restrict oil flow through the bearings. This results in to a $900-$1300 relacement. Also, if you blip the throttle before shutting down, you've revved up the turbo, then cut the oil flow to it. Some people say just listen to the rest of song playing on the radio.

Rene