The new King Kong is going to be 3 hours long.
Personally, we have no problem with 3 hour movies. As long as they move along at a decent clip. I imagine that spectacle such as Kong is a good candidate for working well as a 3 hour picture.
Some 3 hour movies that move well:
The Lord of the Rings films
Some 3 hours movies that don't:
Dances with Wolves
Punch Drunk Love*
Actually, anything by P.T. Anderson
Lost in Translation*
* We know these movies weren't technically 3 hours long. They feel longer. Watching them will take years off your life.