A common myth is that watercooled turbos do not need a cool down, but in reality they still have oil running through them to lube the shaft, so if you shut down to soon you still cook the oil that's held in the shaft, and oil does not continue to circulate after you turn off!

You don't idle for the sake of cooling as such, you idle for the sake of keeping the turbo shaft lubed and to a degree cooled with cooler fresh oil.

30 seconds is long enough if you have not been driving hard, 2-3min if your on the highway or thrashing or just driving fairly hard.
Better safe than sorry I'd say, I give mine at least a min most times.
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Veronica, my 85 760Ti