Chinese

Citric acid will not lose its potency if it is not exposed to any air and is kept in an airtight container.  This probably isn't very feasible in most storage situations though, as humidity is fairly good at getting into the smallest and tightest spaces.  I would recommend using your citric acid within 6 months of receiving it to be on the safe side.  There is nothing worse than making a batch of fizzies and finding them to be substandard in the fizzing department!