This is a great question. I really should pay more attention to the mileage I accrue on my running shoes but I just haven't up to this point. A couple of things lead me to buying a new pair: First, my shoes begin to feel too comfortable, like they are not supporting me well. This is really important to me since I have a compromised right ankle. I need to know that my feet are stable and secure. Second, I begin to feel more 'one' with road and not in a good way! While I've never worn a hole through the bottom of my shoe(s), I have obliterated the tread and it feels like there is very little separating me from the road. When the support is gone my ankle feels the repercussions and this is never good for me.
As you well know, we runners and walkers are a unique bunch so what we need and/or prefer is very individual. I need and enjoy a supportive shoe so when the support is gone, I get new shoes. In all honesty I run my shoes into the ground and should probably replace sooner than I do. With that said, I have a brand new pair at home. I'm going to track this pair and see how much mileage I get out of them before buying again.
BTW, from time to time I've perused on line to see what kinds of responses and recommendations this question elicits. Oh my goodness, the answers are incredibly varied and idiosyncratic depending on experience and personal best practice.