Based on my high school level knowledge of chemistry, I'd say it would last a darn long time in a cool, dry, and dark place. I haven't kept any bark for over a year, but I imagine the turnover rate in whatever basement I ordered my stock from is quite low. It might be a couple years old already, and still seems to abound with goodies.
I don't want comfort. I want God, I want poetry, I want real danger, I want freedom, I want goodness. I want a clever signature.