I never get more than 10,000 miles out of my tires. Regular autocrosses, a few track days and a couple of cross country trips and or some reason the tires are down to the wear bars.
When my rear tires get to this point, they are not safe in the rain, and I replace them. The fronts will have a bit more tread left, but I always buy a set of 4 new tires.
If your tires are aging out, it is proof positive you are neglecting your 911 and not driving enough! Get out there and drive it.