They are good for about 6 months, maybe a tad longer if you keep them in the right condition.
First thing you want to do with your balls is keep them cool, hot balls expand, and then when your balls contract again when they cool down they dont take the same shape, so you end up with odd shaped balls. Also keep your balls moving, if your balls sit on one side for more than a week your balls start to dimple, thats not good for your balls, give your bag a shake every now and again to make sure your balls are ok.
Thanks for the advice. Would room temperature be okay? And how would you recommend storing them, keep them in the bag or in like a cardboard box?