The quick and easy answer is well before they look like these old trainers of mine. However there are more subtle signs before you get to this stage.
- When the millage is creeping up to between 450-550miles.
- When the outer soles begin to degrade and shoe the white inner sole.
- When they are beginning to wear unevenly between shoes.
- The support from the mid-foot or the heel begins to appear far too flexible and unsupportive
however generally you need to decide if the shoe is either not offering you support or if it starts creating asymmetries in running style between side to side.
Enjoy your running and look after your feet