Linda--I think the manufacturers say between 400 to 600 miles, which sounds a lot but isn't really. If any bits of you start hurting that don't normally hurt (hips, knees, ankles) it might be a sign that your shoes are giving up the ghost. If you use your shoes in the gym they might actually wear out faster than in real life (!) because hefting weights presumably puts different stresses on a running shoe than the stresses it was designed for. Still, even a knackered running shoe is probably okay for pottering about in, so we can all retire our shoes and take them gardening or something.
That's a thought: what do people do with running shoes that are past their use-by date?