Funny you ask. First time ive ever seen anyone post about it and coincidentally.. im going to have my exhaust repacked. Maybe. Im haveing mine done depending on how much it will cost because i have holes in my sleeve. As far as how many miles til you do it.. cant answer that but i do know that repacking is supposedly like new. Sound much better when its done and will perform better.
I'm not sure about factory cans, but on an aftermarket can, from time to time the packing [fiberglass] can burn out. The way an exhaust can is made is there is a perforated tube down the center that is wrapped with fiberglass. I had a muzzy on a zx11 that was louder than crap. After I repacked it, it was much quiter and lower,mellower tone. Most pipes you drill out the rivets on the rear of the can, pull the center out, remove the old packing, wrap new fiberglass very tightly and reinsert. You'll need good,large rivets to reinstall the end cap.