For longest shelf life, you need to go for the dry flakes (sodium chlorite), from which you can make the MMS solution.
In a prepper situation, this is not ideal either, I'd think. The flakes won't last forever.
I have had flakes in an air-sealed container, sitting in a dark cupboard at room temperature. It sat there for about 3 years. The MMS solution I made from these flakes came up very low in potency; virtually useless. It would barely activate when adding it to acid.
Since then, when I have bought sodium chlorite flakes, I have kept them in my freezer. The last badge is about 5 years old (been in the freezer all these years), and the MMS solution I make from these flakes seems every bit as potent as when the flakes were new was new.
But if you want to use it in a prepper situation, I guess having it in a frezzer is out of the question.
Just do your best to keep it as cold as possibly. (and keep out of light, too, obviously)