It depends. I live in Portland, Oregon. One defunct open mic I use to go to on a semi regular basis, was a mixed bag, but mostly music...and it went on and on literally til almost closing time, which around here is about 130 to 2 am. So people would generally do their thing and then leave after a while...but at this open mic that was accepted and expected. Sometimes, I'd get up and do my thing at a fairly empty bar....because my name was written too far down...and that would be disappointing...because all the pretty girls would be gone, too. Other times, it was still a full house, and those were my best performances there.

I usually go to poetry open mics, and because generally they have a shorter time frame, and a featured reader (some one who's been booked to read for fifteen minutes to a half hour, usually some one locally admired, or on tour), people tend to stay the whole time.

Like I said, though...it depends on the open mic, who's running it, and the venue.