Although I have nothing on Sasha, I have now gone over 500 miles in a pair of Saucony Grid Tangent 3's, which are a lightweight stability shoe. When I got them, the guy at the running store said I could expect to get about 250 miles out of them because they wer lighter than regular trainers. The shoes still have plenty of outsole left, although the front is more worn than the back. This may be due to a change in my running mechanics, but I haven't experienced any issues with injury at this time. I have ordered another pair from Steve, but Sasha is totally right - how long a shoe lasts needs to be based on how your body is reacting to the mileage on your shoes, although I would certainly add that if you are wearing holes in your shoes, it is probably time to replace them . . .
BTW, Jon is right, too - alternating shoes will increase the life of both pairs of shoes.