|MadSci Network: Chemistry|
If the container remain unopened, the carbonation stays where it is. It needs an escape route if it is going to escape anywhere. There is a weak reaction between carbon dioxide and water to form carbonic acid, but storage conditions or time won't affect this equilibrium. Check out Dan Berger's answer for more info about this reaction. But theoretically, the soft drink will stay carbonated indefinitely. However, the container will typically fail first, after a few years.
If either the closure or the container body is damaged in any way during packaging, storage or distribution (including small dents or scratches), the life of the package will decrease significantly. There have been cases where the acid in the soda has eaten through the can it is in because the can liner was damaged, resulting in a small pin-hole leak. That leak then causes more damage to surrounding cans and chaos reins.
I hope this information helps!
Try the links in the MadSci Library for more information on Chemistry.