When I was running about 70mm lower than standard, my Toyos used to last beween 4000 and 6000 miles but this was due to the ammount of negative camber required to run the car this low. There is a good 4-5mm of tread on the outside, but I have been down to the braiding on numerous occaisions on the inner edge without knowing.
I have to say that they are, along with Goodyear Eagle F1's, the best gripping tyre in the dry I have ever tried.