12 Battles @ 1%.
Reasoning: If it's one battle, I have no choice but to try and claim the drop for myself even in a party of friends because of the time investment (3 hours). In 12 fifteen-minute battles, I have less time invested per battle, so if the drop does come I'm more inclined to pass it to a fellow party member. Also, if I do get the drop, I'm more inclined to stay afterwards for a few more battles to help others, because the battles are relatively short.