I've been wondering the same thing. I typically make mine last 2 years. I have a pair of New Balance that I've glued the soles back on multiple times, have holes in them, etc. You can fee a difference now, as much of the little extra cushions of the sole have fallen off even after gluing on multiple times. I still wear them around town for errands, but not to the office; I wear new tennis shoes to the office.
I do wonder if 2 years is too long and damaging my feet, knees, shins, etc.