Use as quick as possible. If I am keeping it for a few weeks, I tend to decant it into small nags, with a little rice in a twist of loo roll to absorb excess moisture. I also make sure it gets moved or turned regualrly to stop flat spots.
im not too sure on a use by date
but if your storing it make sure you always flip the bags so the balls at the bottom dont get squishe 24/7, flip 'em over
I don't really know a "use by date" but if you are going to keep it for a few weeks or a couple of months it should be fine.
Turn the box about once a week 2 at most. I tend to keep mine near the radiator or somewhere with a contant heat/temperature. change in temperatures is where you are mostlikely to get the flat spots. i'd personally leaved it in the sealed bags so the moisture can't get in. if you do open it i'd shoot it as quick as you can otherwise its useless