I've got an SM58 that the lead singer lost in Nov. and it turned up in the mud in April (after the snow melted, but before anybody broke out their lawnmowers). That was around 1991-92. I still have it and it still works fine, although the steel mesh windscreen eventually got a little rusty and was replaced.
If I find one that doesn't work in my travels, it's almost always because people can't leave things alone. They unscrew the mid-section of the body and break one (or both) of the wires from the capsule to the transformer. I find some that have been dropped so many times the windscreen is almost box-shaped, but the mic will work fine if they haven't gone out of their way to break it.