How long will they do me Dermot? When will I need to change these again? What sort of mileage are in these babies?! These are all questions that I regularly get in relation to the life span of a pair of running shoes.
The fact is that most of the running shoe company's will recommend that you change your shoes post 500 miles of running; after this the foam in the sole unit will become more compressed and so lack the required cushioning which provides the impact protection your body requires. The shoe will also begin to lose its shape and structure which can have a detrimental effect on your running gait. Indeed your gait efficiency and body weight are also factors in determining how long your shoe will last while the surface that you run on is also a factor; for example trial running shoes may last longer by virtue of the fact that the impact force going through the shoe is lesser due to softer trial running surfaces.
There are some obvious tell tale signs that its time to replace your shoe: (1) if the sole unit is worn excessively (2) if the shoe is out of shape or slacking to the side (3) if there is notable wear on the upper or (4) if you're beginning to notice new aches or stains in your body. In general though, you'll know when your running shoe has had its day as it will seem 'dead' of itself and lack bounce. While some people know their shoe is done by the 'feel' of it others record their new shoe mileage on their digital device and so know exactly when they need a change; either way its pretty important to be aware of when your shoe has 'run out of juice' as its the cushioning and structure within the shoe that protect your body from the impact forces exerted when running. While some shoes may not visibly look worn after 500 miles i.e. worn sole or busted toe box, its the compressed foam in the sole that's the problem and the lack of energy return(bounce) from the shoe.
So to sum all of this up; if you run 10 miles(16km) on hard surfaces per week you will need to change your running shoes on an annual basis, 15 miles(24km) will get you 8 months out of them, 20 miles(32km) of running a week and you'll need new ones after 6 months, 30 miles(48km) 4 months and so on, you get the picture!
Bottom line is that when you've 500 miles or 804km ran in your running shoes its time to pay me a visit and use your old shoes for doing the garden!!