mmike I'll make this very easy for you, there was a user Clown17 who believed his sn wouldn't work after opening it because the narrative was that once you open sn, it's now on the clock to oxidize ans very fast, this misunderstanding that it's really that fragile of a manufactured chemical. Now Clown actually nearly died despite the fact that they believed the sn they ingested wouldn't work but it nearly did, that should show that sn is very much not liable to degrade that fast however its not to be mishandled.
My point is, don't be too overly stressed out about something like this when it's been shown that SN is not that fragile and so long as you avoid exposure to moisture, heat sources or sources of ignition as stated in the Safety Data sheet supplied by chemical companies/suppliers then you will be fine, if you feel the need to buy more sn then by all means do it if it makes you feel better.