depends on the 'lee
Mercy is something that, even in negotiated play, is relative. Some 'lees ask for mercy almost right away, and that's patently ignored. Some never ask, but go so far that they pass out. Gotta give mercy BEFORE unconsciousness. 'Lees are no fun when unconscious.
It depends on the 'lee, the negotiation, and whether or not they want a safeword. Having played with folks that don't want a safeword, that can be a real ego trip, and, since I managed to avoid screwing up, was delightful. OTOH, if I had screwed up, I'd be facing legal problems, 'cause it's prosecutable. While consentual play is legal in my state, it's only legal while both parties are in consentual agreement. The moment that your play partner feels unsafe, you're legally cooked. You CAN beat that rap, reportedly, but I'd not want to wager on it.
When you get the chance, though, it's delightful to find folk that just won't cough up that safeword...
blissin' on the notion,
dvnc