At our Irish session in Arizona- usually we play three times through unless it's one of those hypnotic or really monster tunes that demand more- (like Morning Dew or Foxhunter's Reel).

Playing for stepdancers or ceili dancers- we go a few times through- not a set number, but 3 or 4 times through seems reasonable.   

Playing for stepdancers at a feis- sometimes the tune changes for every dancer- depends on the whim of the particular musician.

Playing for contra dances- a medley of 2 or 3 tunes lasts for the whole dance- which means each tune gets played considerably more than 3 times. (maybe 10 or more times?)

Concert performances- we usually play 3 times through, unless it's a multi-part tune (3 or more parts)- then only twice.

Old-time sessions- usually medleys do not apply. (they just play one tune repeatedly until a hypnotic, trance-like state is achieve.

Sharon