I know some people think if a movie is good, it shouldn't matter how long it is, but that has never been my experience. Like, I don't think there would be a movie good enough to justify a 4 hour runtime, for example. I think anything longer than 2.5 hours, it really depends on the pacing. Usually, anything longer than that moves into the "this feels like it's going on for too long" territory.
As much as I love the Godfather, for example, I feel there is a noticeable dip in momentum, and I start becoming less involved. I'm still into the movie, but I become more conscious of the runtime.
One particularly bad experience I had in this regard in a theatre was watching Apocalypse Now, Redux. At 3 hours and 26 minutes, I found I couldn't wait for it to be over, and yet I had mostly been enjoying the movie up until the first hour and a half or so.
Sometimes more is too much.