Hi:
A little late in posting here, but I can always tell my shoes need replacing when my knees get stiff and creaky and also my feet get sore.
I run 3x/week and notice that I get more life out of my shoes when I'm on the treadmill vs outside on the pavement. So, if you do alot of outdoor high impact activity, you might find you need new shoes even sooner.
Take care, Lynn M.