From my understanding if it is kept at a stable temperature indefinitely. Of course this is for pure non sweetened chocolate, but even then I'd be skeptical of decade old chocolate. Honestly if kept in a vacuum I don't see much of a reason why it wouldn't last forever, the only real thing to go "bad" is the oxidation of the cocoa fat. If there are temperature variations you get chocolate bloom, the white powdery appearance on the outside of the chocolate. It doesn't mean there is anything wrong, nor should it really affect the taste.