kiln element question

How long could I reasonably expect my kiln elements to last. So far I have fired

23 bisque firings upto 1000'C 12 eathenware firings upto 1150'C 18 stoneware firings upto 1265'C

I have noticed that at stoneware temperatures I have to be very careful not to overpack the kiln otherwise it struggles to reach temperature in a reasonable time. But I am sure its getting slower.

Thanks

Kevin