I have been thinking alot about this question lately. My four last shoes have been thrown out:
- I had 3 pairs of some Mizuno waves which went a bit flat in the cushioning after about 600 miles so I threw them.
- Then I had 2 Asics gel. I realized one of them was for pronation so I threw them away after 400 miles (they gave me niggles - probably due to pronation shoes). The other Asics I threw away after 550 miles because they went a bit flat. Also, the upper was thin in the forefoot and it began to come holes here and there.
- Now I have some Asics which is close to 600 miles, however they feel fine and I don't have any issues with the cushioning, it feels.
I think I will retire my current shoes out of the rotation when I feel from now on. I don't want to set a certain number. Before, I held the "600 miles formula" for truth, probably because the storekeeper told me so when I bought my some Gel Nimbuses some 10 years ago, which was my first shoes for "running-only". In my school days, I would wear running shoes at school and everyday life and use them also for running in the afternoons.
My reasoning for throwing when it feels right is that, as mentioned, I run quite a lot and it is expensive with good running shoes. Yes, I expect to get 500-600 miles at least from most shoes, but if they feel flat after 400, I will stop running in them and the same - if I feel they last til 1500 miles, the same rule applies.
Is it safe? Well, I am very cautious regarding injuries. And yes, I am stubborn, no doubt. And I am at the same time cautious to spend money, and not. I think I will be able to be subjective around how it feels even if I come to like some shoes dearly. Let's hope so, anyways!
Anecdotally, I remember when I bought the mentioned Nimbuses. Back then I was even more stubborn than now and I had some other Asics Gel (don't remember the model), and I would probably not have gotten the new ones if my mother hadn't been to persistent and bought me my new Nimbuses for my birthday. Of course I felt a big difference in the cushioning. I didn't keep track of mileage on shoes back then - only knew I have had the old shoes for a year - so my guestimate is that they had a bit more then 700 miles taken all use considered (including running). The outsole was almost completely gone, but they looked fine on the upper albeit somewhat dirty (but hey, they were white so what would you expect!). I continued to run perhaps every other run in each shoes (back then I ran somewhat consistently 6-7 miles every other day and long runs (10-15 miles) on sundays.
That was just some thoughts that came to mind :-)