I think the problem stems from telling people how many songs they can do, rather than how long they have got.

The 'Hiring Fair' at Saddleworth Festival, for example, gives each 'act' twenty minutes - including their sound check. Even if they are mid song they would have to stop on the dot of twenty minutes - but people seem to be able to get the timing right, so I haven't seen it happen!

It's fair to everyone - though soloists do have the edge with the sound check, so it encourages the bands to think through what they want before going on stage!