I would say under 400 is fine. if you are using synthetic oil the problem is much less of an issue. Basically you want to let cool oil and air go through the turbo and cool the metal down so the oil doesn't boil itself into a mass of solid particals that can score or demolish bearings.
I find that I by habit park, set the e brake and shift into nuetral and let it idle. By the time I unbuckle the seat belt and turn the radio down so it doesn't scare the crap out of me and crack the windows so I don't boil getting back in, the engine has more than cooled off and I shift into park and shut er down. Good habit to get into.