With the amount of money you'll save on wood you should easily be able to buy a new one every three years if needed.Don't take a personal offence to comments stated by myself as I was just reading the information from their site that states that it's a three year combustor warranty and another three years at a pro-rated rate.
So six years in total but only three real years at 100%.
But there is an extended, 10 year, inspection-based warranty on combustors after 2012. Only problem is that if you need to remove it if it failed, then you need to have a new one to put in until you get the warranty-inspected refund or your old one returned to you without a refund. Either way you're buying a new one.