Check up on your shoe health status!
You need to be concerned about your shoe health. Running on shoes that have lost their cushion can eventually cause injuries. The image above is my Kinvara 6's. The one on the left is a pair with 244 miles on them and the one on the right is a pair with 101 miles on them. You can see the size difference between the two soles. The Kinvara soles tend to "pancake" outward more and more as the miles rack up.
What does this mean?
This means that the sole is losing cushion and the loss of cushion can stress your legs differently. I remember one winter where I was really cranking out the miles and I ran over 600 miles in a pair of Brooks Pure Cadence shoes. I started having trouble with my calves and kept feeling like I had tired legs. I decided to switch out my shoes after a little break and the sore feeling went away. It's amazing what a fresh pair of shoes can do for your body.
So when is a good time to switch shoes out?
Personally, I feel that it differs depending on the shoe. For lighter weight shoes like the Kinvara - I typically like to start to back away from them around 250 miles. I'll still use them as a back up pair but if the shoe is at 250 miles then I'm doing most of my runs in a different shoe. I'll retire the Kinvara completely when it's around 350 miles. For thicker soled shoes like the Altra Olympus - I find they last a lot longer. I have 158 miles on an Olympus and they don't feel much different from when they were new besides some wear on the upper material. Normal wear range is 250 - 600 miles. It takes time to find out what works best for you. I know people who run 1,000 miles in a single pair of shoes. For me, I need the cushioning and responsiveness that comes with a fresh pair. I also find that worn down uppers don't hold my feet in place as well and when my foot starts to slide around... the blisters start to pop up. Good luck experimenting! Let me know what works for you in the comments!