I voted for the 2 hour option, but I'd like to explain that a bit.
Yes, I will watch incredibly long films, and I probably won't even notice if that's just poor pacing or meaningful. But most of the time, I don't. The condition separating the films is just interest.
I don't think there's a problem with going that long. I just think it's riskier. In the fan edit world, I'll probably go after shorter versions of the films that I initially did not like. However, for films that I do already enjoy (like the Back to the Future trilogy and its back to back to back edit) I'll gladly go along for the six hour ride.
A big exception would be the trilogy sequel "squaring" efforts. Even if I don't like the originals, I'm willing to go longer. But again, there's still a somewhat lower chance that I'll be interested in the first place.
Overall though, I don't have any problem with super long films.