lifespan of a shoe?

[VeloVT]

Average lifespan of a running shoe is said to be around 350-450 miles. For lighter shoes or heavier runners, it may be less; for heavier shoes/lighter runners it may be more.

I have never had the discipline to keep "shoe mileage logs" (lots of runners do -- but I typically have more than one pair of shoes going at a time and it gets complicated... ok, I'm lazy), but I can tell when my running shoes need replacing because something will start to hurt -- it might be obvious, like the balls of my feet hurting, or it might just be vague achiness in my knees or even more muscle fatigue. The shoes also just start to feel "dead".

With walking shoes, which might not cause pains that are as obvious since the forces you put on them are smaller, I would examine the shoe -- is the heel counter deformed? Are the uppers deformed? Shoes tilting in or out? If you put your hand in the shoe, are there deep depressions anywhere (under metatarsals, for instance)? Do they feel less springy when you walk, and/or do your feet/ankles feel tired/achy after walks? etc, etc...

Shoes also break down over time just with exposure to air...