Unmilled is key. As long as you keep it dry and cool, it will last quite some time (a year is a completely reasonable amount of time once you get it...in addition to the fact that it is most likely at least 6 months old already). Of course you'll get some flavor degredation as time goes on which is inevitable. Once milled though, you start losing the fresh flavor pretty quick. Just make sure it is covered well so the mice can't get to it and that it is low humidity (below 60% RH is probably right but drier is better).
Event Horizon ~ A tribute to the miracle of fermentation.
Brew what you like. Do this, and you will find your inner brewer.