A very interesting thread. I think it is for sound reasons that a good number of clubs and open mics in my part of the UK promote 2 song spots in 10 mins slots and, less commonly, 3 song spots in 15 mins slots. I know that being able to carry a long song depends very much on the situation, the song itself and the skill of the performer but when songs
go well over 5 mins, it can show a lack of courtesy to others present.