i had a crate of paint in my room for a while, balls were loose out of their bags for about 6 months. would shake it at least once a month. But the balls have warped and don't fit through my barrel anymore, seems like a complete waste :/
So there's no point buying it unless you're going to use it, especially at the end of a season. Just best off buying in bulk (4 for 3) or whatever, then just looking after it properly during season.