rocket mass heater - how long will the drum last?

How long will the top of the 55-gallon drum hold up to the heat applied before you have to replace the said drum?  

Ours has had seven winters and it's still in good condition.

Hi Curtis; Welcome to Permies!

My barrel is in its 6th year and it is good as new.  Barrels have been used over 20 years in some cases. There really is no degradation of the barrel taking place.
If barrels concern you there are other choices.  Brick bells certainly have a several hundred year life expectancy... Water tanks , stainless steel barrels. Lots of choices in barrels, lots of choices in what style, shape ,design ,rmh you want.

Thanks for all the replies to my post. I understand better my choices now in order to build my RMH. I will post my build as I proceed.