as far as I know, none of the stuff in the salt mixes really expires. it's mostly ions, and once those get wet, they don't tend to change much. The main reason saltwater would expire is due to lack of aeration, which you can fix by mixing it, or growth of nasties which might consume things in there. I'd also keep a lid on it to prevent evaporation, and use an opague container so light doesn't get in.
You'd only expect aqueous solutions to expire if they have something with antioxidant properties, or something that's colorful. Saltwater doesn't have either of these things, really, so I'm guessing it's good to use as long as it's kept mixing and doesn't grow anything in it.