it depends on what you want to use it for really. If you plan on using it in tournaments or some sort of important event, then don't keep it any more than a month or two if it's been opened. after this you do find the balls start to deform slightly since they've been exposed to humidity and such. As always the best way to store paint is in a dry place, out of direct sunlight at a normal room temperature. as mentioned make sure you do turn them every so often to try and stop them deforming any further.
On the day, the same sort of rules apply. if its a warm day try and keep your paint out of direct sunlight, especially when its in pods since they basically turn in to mini greenhouses!