I honestly don't think there's anything to worry about. This question has been asked a lot of times and it's difficult to give a precise answer.
When googling the shelf life of SN I found they give it 3-5 years if sealed and stored properly.
Another site said it oxidizes 'very slowly' into nitrate when exposed to air. (See 3.2.9)
Sodium Nitrite | NaNO2 or NO2.Na or NNaO2 | CID 23668193 - structure, chemical names, physical and chemical properties, classification, patents, literature, biological activities, safety/hazards/toxicity information, supplier lists, and more.
pubchem.ncbi.nlm.nih.gov
On an old science forum thread people say that storing SN for decades is not a problem. And that deterioration is negligable.
There's also lots of threads on this forum without any definitive answers.
People use this stuff to cure meat. So it needs to be opened and sealed repeatedly. It's not like it instantaneously converts to sodium nitrate. What would be the use of selling sodium nitrite if that was the case?
Keep it sealed up the best way you can. And test it using the blood test or aquarium test kit when you're ready to use it just to be sure.
And keep to the recommended regime. I feel Stan's guide is well thought through and has been tried and tested.
If any of the links are not allowed, just let me know and I'll remove them.