A very honest statement from manwin
http://nymag.com/news/features/70985/
"Manwin, in fact, has studied the question of optimal clip length. “We tested one minute, three minutes, five minutes,” Antoon says. “The best converting for the content owner is three minutes. The best for the tube sites—for the surfer to come back and back—is five minutes. So we always ask for three to five. We don’t mind if they send us seven to nine.” "