In general workout shoes last under 6 months, unless you don't workout most of the days of the week they they'll last a little longer. My gym in Az did a study on workout shoes and most last about 4 months on average, some maded to the 6th month, I know my shoes last about 4 months. Always check your shoes every two weeks as you could get a defective pair that breaks down faster. Put you hand in there and push down, (you can wash your hands afterwards) and feel around make sure there is some give, if your fingers are telling you it feels hard, your feet aren't getting much cushion, which will sooner or later bother you knees, ankles, hips, and back. Try to toss them before they bother any part of your body, as when they do, your chance of injury goes up by about 45% more then it normally is, so that risk of injury can be really high.
HTH,
Kit