And how do you decide when to retire them?
I used to abide by the 300-mile rule - many years ago (10?).
Then I started running in Newtons, which seemed to last between 4-500 miles before retirement.
In recent years I have switched to Altras, which seems to depend on the model. I had a pair of Ones that barely made it to 300 miles. Then I went through three pair of Escalantes, which each got an easy 400 miles a pair.
For trail running I tried some Newton Bocos, which got over 300 before I retired them for other reasons.
Then I blew through two pairs of Altra Superiors, which seemed disappointingly frail for trail runners.
I look at red wear, but more importantly, soul cushion, to determine if a pair of sneakers is spent.
The Escalantes are my favorite road runners ever. I could probably get more than 400 miles out of them, but like to avoid injuries. (Plus they make nice house slippers in retirement. Trail runners get put on lawn-mowing duty.)
Late to the conversation, but I always stick to the 500 mile rule, unless I determine the shoes have taken too much. If I waited for tread wear, I'd probably hurt myself! Definitely cushioning is my sore spot. If I feel anything significant or a glimpse of pain that's it. I separate shoes based on the length of run too. An older pair may be no trouble for a three mile run, but if I'm out for 10 or more, I want the most cushion possible. I rotate pairs too so that the cushion has time to "plump" between runs. That could sound like a big investment, but I'm always searching sales racks and trying new things since every time I find "the one", they stop making it. The replacement usually is not the same... Also, as new pairs get worked in, I have a good comparison for what the cushion Should feel like since you never notice the decline.
While I do track mileage on my shoes via Strava, like others here, I really don't think there's a solid rule of thumb on miles. I have had shoes from the same brand vary widely in the mileage I could get out of them. For me, it has typically been anywhere between 300 and 600+ miles. I find that, if I listen to my feet and my knees, I go off that to determine when to retire shoes. I also like to rotate through a couple different pairs of varying styles. Yep, I am shoe nerd (as much as I can afford to be anyways).