I run them down until I see belt. I can do that because I have 4 mounted sets. 3 are all-seasons. Nice new tires get bought in Dec and obviously are for winter, alternating on/off with the snows. I also have a "Intermediate" worn set for Spring and Fall. Eventually, a given set gets rotated down to become summer "smokers". I run them down to belts on the hot pavement without concern about wear and if it rains, I slow way down. The hot pavement June, July and August really takes a toll on tires that have decent tread. Cooler/wetter weather makes for slower wear.
So:
Set A- Dec, Jan, Feb
Set B- Mar, Apr, May...Sept, Oct, Nov
Set C- June, July, Aug
Right now, I put on my set of intermediate/summer...Conti Extreme with 3/32nds.
This winter has been wierd, and we now have a SUV w/ new Yokohamas for deep snow.
I'm trying to skip new tires next year (probally won't happen though) I was running Potenza G-009s with 8/32nds but was wasting them this Jan, so I went back to the Contis. I also have a fresh set of Fuzion HRis on my BBS, those wheels cannot be used in winter though.
Anyway, my point is that mutiple mounted sets, even two, makes tire lifespans much more economical. FTR- Audi quattro requires 4 idential size, brand and model of tire.