When I watch a film I'll stop it and make a note of where I stopped. Basically the more sittings, the worse the film. This I've been doing for the last few months now.
Sadly of late this can mean a good four sittings for one film, easy. It often makes it feel like my attention span is poor.
With this being said I obtained a copy of Bicentennial Man, a film I first watched years ago and back then thought it suprisingly good, even though my opinion had always been high of Robin Williams, even after the awful Toys.
With a runtime of 2 hours 12 minutes it's noticably longer than your average film and even though this was now my third time I've seen it, my attention was held completely for the entire duration of the film.