You mean charcoal briquettes? The BBQ type?
Well, if you kept them dry, they would keep just about indefinitely.
Dougwalkabout is right about keeping the charcoal dry. The big danger is that if it gets damp/wet, it has been known to spontaneously combust. Don't think that is a good thing. Aside from the cost, an unintended fire could be a killer too.
Coal is safer.