I don't think you need to worry if you have yours sealed air tight! As someone coming from a lab background, I think people here are very twitchy regarding the potency of their NaNO2, which doesn't diminish if you have it open for a while or if it has solidified. If you think about it, if it would lose potency that easily the standard packaging sized wouldn't be that huge, no one would order 1 kg or 2.5 kg if it goes off as soon as its opened. Then the packaging sizes would be vacuum sealed plastic pouches containing something like 10g, 50 g maaaaybe 100 g.
This is a purely academic question regarding the conversion rate, don't worry! As I said, we even use it in the lab and it has been opened in 2002 and it is still fine and does its job!