I don't mind a long movie if the movie actually engages me during that time. Movies feel longer when they're less engaging. While TF1 and ROTF had similar running times, RotF "felt" longer because it wasn't as engaging. Even now when I watch them on DVD I find it more difficult to watch RotF continuously from start to finish compared to TF1.

So yeah, like 5FDP said -- if the movie is engaging, then you'll want it to last longer. If not, then you'll want it to end sooner. I remember the first time I watched The Fellowship of the Ring in the cinema -- the movie finished with me wanting to watch more! I wanted the movie to go longer, then I looked at the time and realised that it was a 3 hour flick! As the saying goes, time flies when you have fun... so if DotM proves to be a fun movie, then it shouldn't feel long.