Help How long can SN last if I opened the plastic bag and poured it into a plastic jar?

[MOROK]

What the title says. I can order 1 kg of pure 100% SN, but I can't carry the bag to my house. My plan is to open the plastic bag somewhere and store 300 g of it in a plastic jar. Can it be done without SN losing its purity? How long can it last in the jar? Please help!

P.S: I just returned from the mental hospital after surviving my last ctb attempt. I need to be sure.