Well dtrypt commented on having a vial of DMT tincture from 2010 that is still active. You find it in this thread. It was not smoked, and there's no telling mow much if any potency was lost, but 4 years is pretty decent. It at least suggests that it lasts, and does not seem to accelerate degradation. 

It seems like there are three threads that need to be combined. The micro changa one, the liquid dmt one, and this one we are in right now.

As for myself, I did note a definite (though, anecdotal) decline in potency of my traditional changa after a year.

So far, after about 5 months in my freezer, my ethanol DMT seems to be just as potent as before. I'll definitely keep an eye on it, and report what I find.