Yep, rings are only good for around 20,000 revs, beyond that they start to flutter in the ring grooves (move up and down) which wears out the groove. How long it takes to do this I don't know but large ringed engines (.60 size) were used in tether cars 60 or 70 years ago and I suspect they were turning at least 20,000 revs. These cars had no clutch, no throttle and no RC. Here's a [link=http://www.youtube.com/watch?v=T31juxlQuGA]Youtube[/link] clip of a modern tether car doing 200+mph.