Hi Susie,
I have played for some 4 dance medleys with 4 musicians, but nothing longer at a dance. It sounds fun though.
At Carp Camp in Winfield, we often do very long medleys, 30 minutes or more.
At a Summer solstice festival in Los Angeles, we jamming in the hotel late at night, and heard a rumour that all the jams were being shut down. We heard that when a set ended, they would ask us to stop. When I saw the hotel guy standing there, I called another tune, and another tune....we played for another 40 minutes or so...the hotel guy took it very well, was smiling.