I have heard of carbons breaking down, but never seen it first hand. I would guess you are more likely to lowside and grind the cans down before you need to start worrying about the half-life of the carbon used to make the fibers.
I put a carbon fiber MIG slip-on on my R1 about 75,000 miles ago. I've had to repack it twice, but the carbon fiber is perfectly intact. I doubt you are going to really have to worry about it too much.