I did a quick analysis of the stage lengths of the various Boeing's flying tonight. These numbers include pax and cargo, international flights, and some flights within foreign countries. All data is from flightaware.com, and is barely scientific.

Aircraft Avg. flight time
B737-700 2:48
B737-800 3:17
B737-900 3:45
B757-200 4:16
B757-300 4:28
B767-300 6:42