I have yet to repair the end of a synthetic sling's release cord--or at least one that hasn't been constantly striking the ground on the follow-through of an overhand release.
However, they do see wear at the base of the release cord. Fortunately this is easily fixed with something like whipping or leather. The only braided sling I've had to retire didn't have any whipping, and endured a couple months of slinging rough and angular granite before being too worn to safely use.
With natural fiber you not only have to worry about the base of the release cord, but the tassel end as well. Little bits of fiber break off with each shot, which gradually shortens the tassel until it's so short it comes undone during the release.
Luckily this takes awhile to manifest, and all you have to do to fix it is unravel a bit of the release cord, and splice in new strands/fiber as you braid it again. Unless you made the sling from some sort of exotic and rare fiber that you ran out of, this is no biggie at all.
My current braided cotton Apache sling has lasted almost 2 years now, and I don't recall having to repair the tassel end more than once. Then again, that may have to do with the little synthetic-fiber popper I put on the end (like on a whip), and that seems to take a lot of stress off of the tassel itself.
(Pictured below)
The only downside is that if you don't rifle your shot by twisting your forearm/hand like a bowler putting a side-spin on his ball, it can nail you in the finger in a rather painful manner. You learn to correct for this pretty darn quickly.