I don't know about the A330 but on both types I've flown other Airbus and Boeing types with winglets and on each there was something like a 1% fuel burn penalty for a missing winglet. I can't remember if the Airbus could despatch with both winglets missing but is restricted to one absent winglet only. I'm not sure of the exact time limit on the MEL but I reckon it's at least ten days as ours aircraft tended to hang around for a while before the winglet was replaced.