How long will she last?

My RMH is made of 2.5x4.5x9 fire bricks with a old WH tank around the heat riser inside of 55 gal drum which gives me a 2.5" clearance all the way around the riser. My question is how long do you all think the brick will last before I have to replace the burn tunnel and the heat riser our winters are not that bad not like back east but it does get cold at night in the high desert. Just getting the WH cover off of the heat riser is impossible without destroying the heat riser. TIA Azdave