There is no single mileage number that retires every shoe. A better decision combines mileage, loss of cushioning, visible damage, changed comfort and whether new aches began after the shoe became old.
Use mileage as a clue
Many trainers last several hundred miles, but body weight, terrain, gait, shoe construction and rotation all change the answer. Record the date and approximate mileage when you start a pair.
Look for wear and feel
Flattened foam, a tilted outsole, upper tears, exposed midsole material, or a shoe that suddenly feels harsh are useful clues. Compare an older pair with a new pair of the same model if possible.
Replace gradually
Do not debut a radically different shoe at a race. Rotate a new pair into easy runs first, then longer runs, and keep the old pair only if it remains comfortable and structurally sound.