Longevity of running shoes depends a lot on how you are built, how well suited they are to your running gait and the type of shoe. Basically, the lighter a shoe, the less time it will last - running flats are often knackered after 200 miles or so, but this is specialist kit.
A pair of tougher running shoes made for a 100kg pronator will last longer - 500 miles - but be careful you don't have irregular wear - i.e. much greater wear pattern on one side or at the back. This gives the shoes a much shorter life.
I'm just 66kg and have neutral gait. My shoes easily last 1000kms and I tend to have 3 pairs on a cycle. Newest pair is reserved for longest runs, a pair with 500kms is then used for medium length runs and the oldest pair, say 850kms, is then used for short and recovery runs. Any speedwork (intervals or threshold runs) are done in a 4th pair of lightweight running shoes. I just replace them when they feel dead - usually about 600kms.
This shoe rotation means I tend to avoid lower leg injuries. Cheers, TD.
