My rule of thumb is 600 miles or 6 months, what ever comes first. Not that different from anyone else here.

What I wanted to add is that running shoes get compressed while running and don't regain their maximum cushiness until at least 36 hours later. If you run on them sooner than that you will break them down faster and it's not good for you. So people who run daily should have at least 2 pairs of shoes on the go. Because I tend to be on the conservative side when it comes to things like injury prevention, I would be tempted to follow the same guidelines for walking shoes made with the same type of EVA midsole as runners.