I cannot speak to this directly. But I would assume that storage temp does not affect DNP. I would especially think that cold storage temps are good. And you can test a small amount of your DNP at any time by lighting it on fire (safely away from your DNP store). Good DNP will explode like gunpowder.
Anyways, what type of DNP do you have? Is it crystal or powder? Is it already capsuled or raw powder?
I have made my own caps of DNP (very messy! do outdoors). Was able to find DNP crystal for about $1 a gram on ebay. Now its not as easy to find on ebay. But way cheaper than buying caps, and you can trust your dosage since you weighed it out and made it.