How long do firebricks last?

My Alderlea T5 insert was put in in 2011. I burn nights, weekends and holidays. A number of the original bricks became pitted and chipped, so I decided to replace a few with some from Tractor Supply. Well, by the time I got the old ones out, I wound up replacing them all; they seemed so brittle that some even broke as I was extracting them.
The new ones fit very tightly. The fun part was chisel-cutting the odd-man-out, and rubbing a few across the concrete outside to file them down to fit. Don't know how long they will last, but they sure are nice and smooth.

 

Thats the way to do it boys! Ya gotta take care of them.....