As said depends on the string material and the maker. I make my own so know when I make a 93" string it will be 93" next year when I change it. I have also seen strings that are stretched up to 3/4" but since I don't know the length when it was put on (some strings vary a lot from what is called for) I can't say how much was stretch and how much was just a poorly made string. Using 450+ I don't notice any change from initial making through sighting in and beyond. I also recommend changing a string every year as a string takes a lot of abuse, so why not be safe with it?
Moun Dave, where are you at? I'm in Erie. If you're near North East my friend has a shop (Dave's Archery), stop in and we can shoot the bull.