The standard answer is 300 to 500 miles. That's fine as a starting point, but it's a terrible rule if you weigh 120 pounds or if you weigh 220. If you run on pavement or soft trail. If your shoes sit in the sun all day in your car.
The better answer is to read the shoe.
Heel wear
Flip the shoe over. If the outsole at the heel is smooth and you can see the midsole foam, it's late. Some wear is fine. A bald patch that's wider than a quarter means the shoe is changing how your foot lands.
Midsole compression
Press hard on the midsole. New foam bounces back fast. Dead foam feels soft, then stays compressed for a second before recovering. A 30-mile shoe and a 400-mile shoe feel noticeably different here.
The twist test
Hold the shoe at heel and toe. Twist. A new shoe has some give but pushes back. A dead shoe twists too easily. That means the midsole has lost structure and your feet are working harder to stabilize each step.
Your body
Niggles that weren't there before. A sore knee, a hot spot, a calf tightness. Shoes aren't always the cause, but they're worth checking first. Rotate in a new pair for a week and see if the niggle fades.
My rule
Two pairs in rotation. When one feels dead, I retire it and buy a replacement. I buy before I need. That keeps me from running too long in a tired pair.
We see a lot of runners come in saying "my knee hurts." We always check the shoe first. More often than not, it's the shoe.
