I heard a Captain do this one as a part of his pre take off briefing.

"Ladies and Gentlemen welcome aboard XYZ Airlines. We would like to remind you that smoking is not allowed aboard this flight. Our flight time today should be 1 hour and 45 minutes. If we start to land significantly sooner than that go ahead and burn em. We have 2 and a half hours of fuel on board, so if we fly longer than that go ahead and burn em as well."