It varies from product to product, but you can expect at least 18 months for some types, a couple years or more for others (the resin often outlasts the hardener). If you’re concerned about whether old epoxy is still viable, just mix up a small batch in a plastic cup and see if it cures. The thing most likely to cause a problem is cross contamination, which means tops or plungers can get stuck. Sometimes running them under warm water can help free things up.