I guess this depends on the sub, but for me it would be when they completely stop caring about you at all, and only use you for themselves. This can be fun at times, but when it goes so far as to maybe never (ever) receive any aftercare, or only so much as to make sure that you don't leave them.. things alone those lines. But there are people who are into that, so their answer would be different I suppose.

For me, it's about trust. Trust and the perception that my "sexy dominant/controlling asshole" Master always has my best interest at heart whether or not I can direct results of it. Trust & honest communication form the basis of our D/s relationship. Nourishing and protecting this trust continues to allow us to push limits that would (in any of the 'vanilla' relationships I've been involved in) foster resentment rather than foster growth. Perception is key. Trust and faith in my Master allows my perception to venture into uncharted territories of unconditional love and service.