There are earlier threads that discuss this problem. try 'Singaround Etiquette'.

Not an easy one to solve. As a free concert organiser I have the same problem. How long do I give to duets, quartets, solo singers? Generally I give established groups of any sort about 20 mins, floor singers one song, floor singer duets 2 songs and so on on a 2 or 3 hour concert, but even this can cause problems. Solo recording artists normally get a full 20-minute spot.