I only know for sure that it's time to shut up when I notice the glazed look on other peoples' faces...or when I stop talking and all of a sudden everyone seems to be finding other things to do or talk about. Kind of like being a real-life/real-time threadkiller, I guess.
The alternative--and this actually works pretty well--is to feel as if you are purposely leaving stuff out and just wait to be asked for the rest. Same with email. Tends to lead to editing oneself into complete silence, though...
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Not all who wander are lost...