Bilstein 5100 Question

I have Bilstein 5100's all the around and was wondering when would be a good time to replace for new ones? I have had mine on since 2010 or 2011.
Also, my left rear shock rubber boot has torn, can it be replaced?

 

These should last quite a few years, even with moderate abuse. Don't worry about the boot. It does nothing. You could remove all of the boots for a cleaner look.

 

is it possible to get new boots to replace on the rears?

 

Im sure you can get a universal boot for cheap. And yea my 5100s on my tundra were on for 30k and abused and never wore down when i replaced and ive had them on the taco for about 30k also. Not worn but about to replace for higher quality ones.

 

i thought they had a lifetime warranty?