Sorry Boo, I must disagree... that time scale only works if the cork is EXPOSED to the elements..... Marg0 specifically asked about *unexposed*.....
I would guess at least double what Boo suggested for an unexposed cork.... but it would depend on whether it had been used or not... and what it had been used for.
you'll probably be safe from it decomposing for ages, but if you where to eat loads of baked beans and then bend over you'd make a pretty good human(?) cork gun.