There's no need to go through a 'cool down' process on a TDi. They don't have anything like the amount of thermal problems that a petrol turbo engine has. So no, a 10-20 second spool down is all that's needed as the turbo still spins at 125K RPM, and faster under load.
There's practically no way to drive a TDi off boost either, without coasting, but then even gentle acceleration into a side road or cul-de-sac will make a TDi's turbo spin-up.
All that said, I always give it 10 seconds minimum to let the turbo spin down.
Incidentally, if the car is left idling for over a minute or two, the turbo starts howling again. It's definitely ECU controlled and a quick tiny blip of the throttle stops it. Is the intention to lean out the exhaust gases or something?