Road running will make your shoes wear faster than trail running, but the way you run is an even bigger factor. A heavy heel-striker who runs exclusively on trails will most likely wear out his shoes well before a biomechanically efficient road runner. If you're worried that your shoes are too old, take a close look at them. If the upper appears pulled or stretched so that the foot is sliding off the midsole, or the grooves on the outsole are worn smooth, it's time for new running shoes.