As has been said, film comes listed with an expiration date, however if kept cool the film can last amazingly long. I keep my film inside the plastic container to keep condensation out then I put my film in the freezer.
As you know, Emulsion is a chemical mixture, and like all chemical mixtures, there is a constant chemical reaction taking place, albeit a very slow one; that is the Silver Nitrite breaking down to Silver Nitrate (or something along those lines). Basic chemistry class tells us that heat speeds up chemical reactions, and by removing heat, chemical reactions slow down.
So in short, stick your film in the freezer
And all at once the crowd begins to sing: Sometimes the hardest thing and the right thing are the same