If the O-ring idlers[1] on The Red Baron are anything to go by, the wear of the O-ring is determined primarily by how much bouncing around the chain does. After about 6000km the one by the fork crown has several chunks missing, whereas the one under the seat lacks any significant damage. (The drive-side idler has teeth rather than an O-ring, and is like new, but dirtier. I can't help thinking that passing roller chain over a smooth pulley is fundamentally silly.)
[1] Alligt 50mm.