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It depends on your load, speed and ambient air conditions. Lately it's been in the high 90's to 100 degrees out and it takes about 1-3 minutes to cool to 300*F or under.
I just installed a turbo and pyro gauge set up in my f250,usually as soon as I idle I get to about 300 or so degrees, but what you should be concerned with is hot oil in the turbo, so if I use my turbo alot I give it a few minutes, if I use it once or twice on the highway I let it cool for like 1.5 minutes.