On the mod and RnB scenes 30 minutes is the standard DJ set with people doing 2 sets, one early and one late. It works well as it usually gets a good mix of sounds. Ive been to too many nights where people play for themselves and dont care if anyboby is dancing as long as its rare and they can show off their collection. Atleast with a 30 min set you dont have to put up with them for too long!