Tips for storage
1. put any opened paint into Ziploc (that's resealable sandwich bags for you Brits).
2. store in the least disturbed, most temperature stable place you can find. Places such as that include under the bed, top shelf of the closet, bottom shelf of the pantry, next to the Pope's condom stash, etc.
3. every week or so, pick up the bags and GENTLY roll the paint around a bit to prevent it from settling too much in one position.
4. store in this manner IMMEDIATELY after purchase and/or play.
Exactly how long the paint will stay fresh from there depends on the brand. You'll find out from experience as to which will last longer; your local results may vary from mine.