Infinite shelf life.
According to Pubchem, SN will slowly oxidize to sodium nitrate in air. sodium nitrate is lethal but with a higher dose. I don't know whether it's peaceful or how it works. sodium nitrate probably needs more time. idk...
You should also keep your SN dry.
If you have opened your SN bottle, you can throw away the top of your bottle that was mostly in contact with air, then close it tightly. if you haven't opened it at all, it's good, don't open it. Put SN bottle in a plastic bag with moisture absorber silica gel. try to remove the air from the plastic. Keep the plastic in a dry place.
