I know someone exactly like the organiser Bonnie mentions - though I am sure it is a different person, as I don't think Bonnie has performed at his club.

I'll make an allowance for an overrun of 2-3 minutes (i.e. less than the duration of a normal song) due to slight misjudgement. Anything above that is intentional in my book and inexcusable; in a professional context, it is also unforgiveable.

Having said that, I have found most performers to be very considerate in this respect, and most will err on the side of caution. A typical such example was at the Association of Festival Organisers conference recently, during the FolkWise showcase concert: There were 5 performers, each allotted 15 minutes, with an extra 5 mins for the changeover. The first three performers were so careful to avoid overrunning that they "gave away" almost 15 minutes between them - and the organiser then had to ask me to extend my spot a little, or people coming to see the subsequent acts would have found that they arrived too late!.