One method; download the electronic timetable from ual.com. The .pdf version has the flight duration in the far right-hand column much like the old paper timetables.

http://www.uatimetable.com/United.pdf

The Windows version of the electronic timetable has the same information, albeit in a slightly different format.

http://www.ual.com/site/primary/0,10017,1891,00.html