Like anything you're going to get varying answers. My first touchscreen (a lilliput) lasted about a year and then stopped working. I'm only about 6 months into screen #2 (a xenarc this time around).
Xenarc's website sets life span by the number of screen touches. "Life span of 35,000,000 touches versus 1,000,000 touches from 4-wire resistive" I have no idea if that's really the case.