Back when I was using conventional running shoes, I'd end up retiring them from running somewhere around 500 miles. But really, it was a few cues in my body that would let me know it was time. If my many-time sprained ankle was aching when I woke up in the morning, I probably needed new shoes.

(Now that I run in VFF's, ankles almost never hurt. My first pair have 1300+ miles on them, newer pair 700+, and I still run in both every day...)