How long can you keep chocolate, and what is the best way to store it?
Regardless of type, all chocolate should be stored in a cool and low humidity (dry) place away from direct sunlight. It would be best to seal it in an air-tight container, because, as ElendiTheTall said, the cocoa butter in it will absorb flavors.

Dark chocolate will last for years. Milk and white chocolate will last for a much shorter time (a few months), because of their milk content.

Improperly stored chocolate will develop bloom, which shows as a white or grey streaking or spotting on the surface. The spotting or streaking is cocoa butter fat separating and is a sign that the chocolate's temper has been lost. This kind of chocolate is still suitable for any application where the chocolate will be fully melted (most baking). It can even be used as the base (non-seed) chocolate for tempering with the seeding method, but it should not be used for other candy making.