Most of the time I dont like looking at the clock, I just go with the flow. If it takes 6 hours, I dont mind....if it takes 15 mins...I dont mind...as long as its fun. It really depends on the people.
I guess it would only be a bad marathon session if one person keeps going and going and the other person is bored or wants to stop....but that should never happen, because people should know when to speak up.