Replace your running shoes every 300 to 500 miles or 6 to 8 months
Added
Keep a rough log of miles on each pair of running shoes and plan a replacement before they're worn out. The American Academy of Orthopaedic Surgeons says how fast shoes wear depends on the material, your workouts and the surfaces you run on, and the more you work out, the sooner you'll likely need new ones.
The result
Result: AAOS OrthoInfo: in general, replace workout shoes every 300 to 500 miles or 6 to 8 months, whichever comes first.
Note: Not medical advice. Talk to your doctor before changing your training, diet, or supplements.
Source
AAOS OrthoInfo (Safe Exercise) (opens the original source in a new tab). Check it for the latest numbers before you act.