Same here. I tend to purge often so if the smoke goes south, it's over. If it's great I've stuck the end in a cob and finished it.It varies a great deal for me. Basically, if time is not an issue, I smoke it as long as it's good. If I'm not enjoying it, then there's no good reason to continue on.
It varies but I usually pitch it when the smoke starts getting hot. It differs from cigar to cigar but generally it's around the last 1-1.5"
I've smoked them down to where my fingers have gotten burned, and others I'll put out by the last third. All depends on the cigar, and my mood.