Assumptions: You just parked. How long to idle down (180F thermostat, good radiator, properly operating, cooling system, and engine)...

After typical use:

After typical use, that included a hill climb, steep grade, etc. - After 2-4 minutes of cool down:

EGT	TWT	Idle down time...	Turbo's status& Extra things...
900	above 220	4-8	Still Glowing brightly (seen in total darkness) Try idling up to 2,000 RPM's, for 1 minute or so...
900	above 200	2-4	Glowing. Try idling up to 2,000 RPM's, for half minute or so...
Below 900	below 185	1-2	Probably not glowing...
About 800	Below 180	1	Probably cooled down...

 Typical driving temps:

EGT	TWT	Idle down time...	Turbo's status
1275-1325	around 205	4-8	Typical (up hill) hi speed mountain driving temps.
1200-1300	around 195	3-6	Typical highway driving temps.
900-1200	around 185	2-4	Typical in town driving temps.
above temps	150-180	1-8	Till EGT/TWT is compliant...

What I'm trying to say, is that the EGT, is not the end all test. I thought it was for determining weather the turbo is cool enough, for me to shut off the engine, it's not...
I saw that the EGT could drop after 2-4 minutes, but the turbo water temp (TWT), may rise to 210-230F (the turbo is still glowing hot (seen in total darkness (As in somewhere there are no street lights, moon light, etc.). I think, this indicates that the turbo is moving the heat to the coolant, but the coolant has not yet, completely removed it. This is why I recommend idling up to 2,000 RPM for one minute or so. Idle normally for one more minute, if EGT is below 900F and TWT is 195F, or dropping great! If, TWT is still high (above 210F), then 2,000 RPM again. Otherwise just idle it some more...
I've thought of adding an extra turbo water cooler (a trany or oil cooler) and/or a thematically controlled electric pump, to help do some extra cooling...

What is TWT?
Turbo-Water-Temperature (I made it up. Maybe It should be called TCT?). The water (coolant) line, that come out of the turbo and is connected to the upper part of the radiator, is the water outlet (from the turbo). I think this water temperature is a vary good indicator, of how hot the turbo actually is, and the cooling system. Especially when, compared to the EGT (Exhaust-Gas-Temperature (also know as a pyromitor).
To install TWT monitoring, get a coolant temp meter (100-250F). I like Auto Meter. Get a large "T" that will except 3/8 barbed fittings, and the necessarily adapters needed. Get sum 3/8 hose, 2 ft long. Remove the factory hose, that is connected to the radiator, to the metal turbo pipe. Make a loop that has the "T" where you want it.

• DO NOT PUT THIS SENSOR NEAR THE TURBO, IT IS TOO HOT THERE!!! 
• Use Teflon tape.
• Solder, tin the wire so it wont corrode.
• Use an adapter as a spacer so that sensor doesn't block the coolant passage (make sure you can see through the "T."