Tires wear out when they wear out. How and where you drive and whether or not you rotate the tires will greatly effect tire wear. High speed cornering and "burnouts" will greatly accelerate wear. It is generaly accepted that, if you drive the car very little, so that the tires do not "wear out" that they be replaced every 5-7 years. Here is a link to an article by Tire Rack on the "life" of tires.