Internet wisdom suggests that 'running' shoes should be replaced after around 500 miles. Would this figure be the same for trail shoes? They may get more damage from roots, rocks and the like, but they wont be getting the same level of pounding that you get from running on tarmac/concrete. Road shoes are divided into three levels of pronation, while trail shoes generally are not. This suggests that the shoe (and your feet) get less of a hammering off road and should last longer?