I've never come across an absolute limit, obviously as you remove material from the cylinder you eventually reach the point where you can't get a big enough piston. For a cheaper engine such as a Rotax you'll end up scrapping it, but for something more expensive (a Merlin for example) you'll probably re-line the cylinder if you really have to - but that isn't going to happen very often because it's a very expensive operation.
So, no absolute, basically you'll scrap the engine when you get to the point that it's cheaper to replace it than the alternative. I've certainly come across engines that have had at least half a dozen overhauls.
G