I'm a fanatic about my shoes. When I find a style I like, I buy up every pair in my size I can find in the city. When I start wearing them, I take a sharpie and put the date somewhere on them.
I wear them for workouts 3 months, maximum. Then I wear them just around until they wear out or I'll give them to someone who can't afford any. There's a chick at our gym who has worn the same shoes every day for two years and walked 4 miles in them daily!!! And she's one who could afford new ones. I don't understand why people would risk injuring themselves out of cheapness.
I learned about alternating days with shoes, too. There was a seminar about it at the ACSM, and it was not sponsored by a shoe manufacturer, either. It seems it takes a full 48 hours for the cushion to completely bounce back after a lot of impact.