Salt...

[Seagars]

how long does it last before it goes "bad" in some form of storage?

 

[brmreefer]

I would think indefinite as long as it is sealed properly and moisture is not being introduced to the salt.

 

[Payne Train]

Keep it sealed up and it should last quite a while I would think.
Keep moisture away from it.

 

[Randy Holmes-Farley Verified Community Expert]

The long term issue if moist is precipitation of calcium carbonate. Mix it up and let it settle clear, then measure alkalinity. If that is OK for you, it is good to go. If not, you can always boost it and calcium back.