Personally now that you know how long it takes I would say never join one unless you know you have time to complete it.
For example if you have 90 minutes to play and you know a duty takes 60 minutes than after 30 minutes you should cancel your duty finder.
Lets be honest, whats the point of running a dungeon if you cant finish.

All that being said, if I was to start a dungeon with limited time I would let the party members know as soon as I joined, if they asked me to leave because of it I would.
Surprising them with "oops I gotta go" and then bouncing is pretty crappy.