Depends on driving habbits. If you have good turbo driving habbits and stay out of boost for the last 2-5min of your driving, then you could shut it down 15-30sec after parking your car.
If, on the other hand, your in boost all the time, but still fairly regular spirited driving, then I would say 2-3min before shutting it down.
If your a maniac, and your WOT
all the time and run on the ragged edge and pull into your parking spot Full hot and just out of a WOT
blast, I would say let it cool for 5 min before shut down.
Also, the larger the turbo, the faster damage will occur from this. (ie. a small turbo like the mitts on our cars can suffer hot shut down and coking and still function relatively well because there isn't enough weight on the shaft (turbine wheel size) to cause serious problems. As the turbine wheel/shaft gets larger bearing failure become more imminent and a really hot shutdown can bend the shaft and end with the turbine wheel exiting the turbine housing)
After every pass at the track (39psi last fall) I let the Charger idle for 3-5min back at the pit or if I'm back in staging before shutting down. Now this may be over kill, because I have the whole trip back down the return road for it to cool off out of boost, but like I said, never had a turbo failure, so why change a good thing :)
Everyone should be using synthetic oil by now And keep your oil Clean!