I quit smoking a cigar when it stops being enjoyable, for whatever reason. If it's just a nasty cigar, I'll quit once I'm sure it's nasty and it's either not going to get better or it's not worth powering through the nastiness to get to an uncertain payoff. If it's a normal (good but not extraordinary) cigar, I'll smoke it until it either starts to get bitter or too hot. Usually that's with somewhere between 1 and 2 inches remaining. If it doesn't get bitter, I'll smoke it until I can't hold on to it any longer, but that doesn't happen very often.
I honestly can't think of a cigar I've stopped at only halfway if I enjoyed the first portion. It seems that once they start out (or become) good, they stay that way until they get near the head.