Fire brick burn chamber life span?

What is the average life span of a fire brick lined burn chamber (J)?  I see some folks tearing their burn chambers apart and rebuilding and that makes me nervous.  I don't want to rebuild after a short period of time (five years).  Ideally I'd like to build it once and not have to mess with it again in my life, but, is that realistic?

Hi Tim!  Welcome to the Rocket Mass Heater Hub of the World.

Right out of the gate I'm going to say that to expect a lifetime guarantee on firebrick, whether its in a box stove or RMH is not gonna happen.
Even though they are designed to handle very high heat and go through many heat/cooling cycles, inevitably over the years they are going to crack and spall.
I ran a J tube RMH for many years using firebrick splits and saw how harsh that high heat was on them.
As you may know, many box stoves are designed to have the bricks be replaceable.....a bit harder to do with a J tube without a bit of surgery.

Another alternative though would be to use a product called ceramic fiber board.
An example: Rocket Stove Cores

I have not heard much about them loosing their integrity over time other than through abrasion so this may be what your looking for instead of firebrick.

Let us know if you have any further questions to getting your dream stove started.

Hi Tim;
Gerry was spot on.   All wood stoves that use firebricks will need them replaced after time.
How much time? Well in a metal box stove, run hot  I have seen bricks crack in less than a year.  But even cracked, as long as they do not fall out. Then folks just keep using them.
This of course lets the high heat attack the metal skin of the box stove, causing it to start warping.  Then when new bricks are finally installed. They do not fit properly, allowing the heat to continue attacking the metal. After 5-10 years the stove is scrap metal or relegated to an unheated outbuilding for occasional use.

You haven't mentioned what style of RMH you were thinking of.
As Gerry said the ceramic board J tube cores are awesome , although they use split bricks in the feed tube. Those bricks are easily replaced WHEN they do crack.
Rocket mass heaters are developing very high temperatures very quickly. A hard product like a firebrick can only take the heat cycling for so long before they crack.
The same holds true in an all firebrick core when it comes to what mortar you want to use.
An expensive refractory mortar would seem like the product to use...  it is not!  It will absolutely crack  and fall out... leaving your bricks ready to fall out as well.
A mortar of fire clay and builders sand is the product to use in the highest heat areas.  A clay sand mortar can flex, a refractory mortar is ridged and can not move, it WILL crack and fall out.
When clay sand mortar starts to fall out, a wet finger with new clay mortar will fix the problem. No fuss very little mess at a cost of penny's.

As a newbie to RMH's it all seems like a huge scary job... and then you hear how we all rebuild our stoves from time to time... more scary!  All that work and now it needs rebuilt!!!
When you decide what style RMH you want here is a tip.   Do not try to overthink the whole project!  You will end up stalling while you worry about what comes next.
It is hard to do but... Focus on one aspect... finish it then move on to worrying about the next step.
We call this rocket science. It is, as far as the innovators with specialty testing equipment who pioneered the development! They did the hard work!  
You and I just need to follow in their footsteps.
After you build your first RMH you will look back and wonder how you ever thought that building one was hard.  Its Not.   Bricks and mud a caveman could do it!  If that caveman had bricks that is...