From the Burning Questions chapter of the 2nd edition:
"I've never seen one burn out, though we haven't run the same one for more than twelve years. But with all the oxygen used up, so long as you keep it dry, it can't burn from inside nor rust from outside, so it should last a very long time."
As long as you are not condensing the vapors within the barrel, it is similar in construction to a commercial heat exchanger and 20 to 30 years is quite possible if set up right. If you choose to get the top very hot for cooking and such, it will indeed oxidize at a faster rate than that and you might be in the 10 to 20 range.
Professor of Thermal and Electrical Engineering, Welding/metallurgy: Licenses: PE license, Mechanical license Variety of other "certifications" from industry groups such as Refrigeration Service Engineers Society http://www.rses.org/, ASHRE http://www.ashrae.org/ Ect.