I've got stuff around here that's no telling how old. I've got a bad habit of keeping anything that still works, but every time we pack up to go to a full fit show, we take the fullest cans and put the emptier ones back on the shelf. Once or twice a year I go through and try them all - if they spray, and it looks and smells and feels like it should, I put it back on the shelf... lol. Like you, we go to way more shows where we don't fit than where we do these days.
But back to your original question. I think, especially if you try to keep them from getting really hot or really cold, most of it's as good as new for a couple of years at least, maybe alot longer.
Oh, and if anybody wants a bunch of 1/4 full cans of assorted adhesives and what not, let me know...
