A pair of mid-high range runners should last for 1000km. If you run 52 weeks of the year that is equal to approx 20 km per week.  If you are running/walking more than that per week you need to replace your shoes sooner.

Looking after your feet:

1. Mark on the calendar when you get your shoes and keep an eye on it. I find if I keep my shoes past their use-by date I am more prone to injuries.
2. Go and get them fitted properly because the type of foot you have determines the type of shoe that is right for you. I overpronate so I need lots of support however with orthotics my orthoses (inserts) I wear a neutral shoe.
3. Consider a visit to a podiatrist/orthotist if you seem to be getting lots of foot/leg injuries. You only have one pair of feet to last you a life time - it is important to look after them.

I am wearing an Asic runner at the moment but I have tried many others that I like. Working out the best shoe for you is a personal journey. As the photo above shows I work my shoes hard. Good runners make running easier or so I keep telling myself.