I've got some epoxy from a previous project - how does this stuff hold up?

Epoxy has a very long shelf-life if kept at room temperature. Years and years. The hardener always turns dark, no matter which brand it is, but that doesn't seem to have any chemical effect, nor any noticeable effect on appearance.

The exception is if the epoxy has gotten frozen, say in a cold garage or shed. Epoxy resin that has gotten cold can crystallize, which you can fix by simply heating the jug in a bucket of hot water. The resin reconstitutes. But in my personal experience epoxy that has frozen doesn't cure, and should be replaced.

I get melancholy and tearful every time I see the four-gallon unit of resin (big $) in my basement that got frozen, and subsequently ruined a boat I was building by not curing. But I just can't bring myself to throw it away.