With the amount we pay for these shoes i tend to make sure i wear them to the death.
When i first started running i would change every few hundred miles and just ended up with lots of shoes and not alot of cash.
When i actually sat back and thought what bad effects part worn shoes were having on me i struggled to find any. I now wear them right out untill the sole wears off or the holes become to large in the uppers.
At the moment i have three pairs i regulary use. My trail shoes are Hokas which i have just changed because my others ripped open during the Thames trot last week. They were six months old. My roadies are Mizuno wave inspire i've had over a year (don't do very much road running). I also use an old pair of Roclites in very muddy conditions, these are knackered now and probably 3 years old. I also bought a couple of pairs of Brooks trail shoes last year that are barely worn.
I ran over 2000 miles last year.