Like some other here, I use my own best judgement. After a hard drive, I always take my time gathering cell phone, putting away Garmin, etc. to let it cool down for 30+ seconds. But after a normal drive, if I cruise slowly thru my neighborhood to get to my house, I consider that my cooldown time- probably a better cooldown than sitting still in the driveway because you're getting a 20mph "wind" assist thru the radiator.
But if I pull into a parking lot or other situation where I did not get the "cruise time" to let it cool, I will make sure I give it at least 15-20 seconds at idle before shutting down.
I had a Turbo Timer on my Supra, and I just didn't like it. I like to park my car in gear, which you are technically supposed to do. It was hell to break myself of that habit, and I eventually just ditched the TT, realizing that it didn't take that much time out of my life to let the turbo cool.