Never done a "Battle of the Bands" affair (dislike the concept intensely), but I have played quite a few all-dayers, which is basically a similar format but without the "competition" element.
Turn up early, find out where on the bill you're playing and how long the set is (and the "turn-around" time), watch sound check, watch preceeding bands, play set (30-45 mins), watch the rest of the bands, get food, catch up with old friends, talk to other bands & audience members, sell some merchandise (CD's/shirts/etc), etc.