A perhaps cliche oversimplification would be to say, "when it is no longer safe, sane or consensual."

Consent is funny thing, and can be given or withdrawn very subtly..and sometimes, as above, should be withdrawn and isn't...two way street there..

BDSM is often, (in my humbly limited experience) a very fine line to walk, right out on the edge of the cliff...I can see that it takes very strong people
walk out there without going over the edge, at least not without a rope <WEG>. We often do not credit our pioneers enough..

maybe I didn't say that too well...