If we are talking role play with a partner they I would say the line is wherever one of the two participants becomes uncomfortable or is no longer having fun....fun being the object I would think.
Personally I have few lines and generally edit myself according to the person I am chatting with, role playing with, etc. My fantasies can admittedly extreme and if unchecked will usually include snuff. Weirdly enough, I don't care for beating and torturing all that much especially to the point of blood. Raping a bloody bruised body doesn't do it for me! The snuff only comes into play as an ending and is most usually in the form of some kind of asphyxiation. There are some more extreme things I have talked or thought about that would definitely result in blood but since it is only fantasy, I can unreasonably overlook that fact and not think in those terms. The only no-no I can think of is anything that has to do with shit!?!?
lol
That detailed enough?? lol