I wear mine for walking and to work.
When my body starts to ache on my long walks (4+ miles), then I replace. Sometimes 4 months, sometimes 6 months, sometimes a year - depends on the shoe. The $160 pair that I got last time has lasted 2x longer than the $50 pair I had before that. And still going.
So if your feet, ankle, knees, shins start to hurt - replace.