From what I've read (any experienced soaper can correct me if I'm wrong), it doesn't go bad but different climates/atmosphere changes can affect the consistency of it.
Humidity can make it clump and tough to use...same goes for hot or cold temps.
I had some lye at the bottom of a five gallon bucket that was pretty old. It soaped just fine, with the exception of a few clumps that I sifted out. You mainly want to keep it cool and dry. Moisture is the enemy! :shock: