I just bought a 2000 box of Toxin paintballs (along with a new marker for the kid for Christmas

). I've heard different things about how long the balls last. Last year the paintball field we went to told us they wouldn't last 6 months. Other sources says that, if stored correctly, they can last over a year, even 18 months.
I will be sharing this box with my son, and friend, so there's really no telling how long it will last. My friend and I seem to be pretty conservative, I didn't even finish a 200 hopper in the 2 hours we played. And that included my son borrowing my marker. Based on our schedules, play frequency will probably be sporadic.
So, if I store the balls inside the house, how long can I expect them to last? Does the expiration change once a 500 bag has been opened?
How would I know if they are bad, and I should not use them?