Manual says 30 secs. I use my own theory. I have a boost gauge installed on my 7. Usually when I am driving to a place and pulling into places to park, such as my driveway, I am using no boost as per my gauge and I shut off my car right away. I figure the idle time as I'm coasting down the road to my driveway, or easing into parking spaces is about 30 seconds of no boost. If I was hammering on the car and was in boost for a long time, I would def not shut off the car right away, but because I can physically tell I am not in boost as well as coasting into parking spots or into my driveway (little throttle use), that time is typically enough for the turbo to cool down.
Not sure if Im doing any harm, but since I am not in boost for at least 30 seconds while performing those maneuvers, in theory I should be fine.