If a movie is longer than 150 minutes, it probably shouldn't be.*
I believe this for many reasons.
--Alfred Hitchcock said the length of a film should be proportional to the size of the human bladder. I couldn't agree more. Movie theaters should have intermission for movies longer than 150 minutes. We don't expect people to watch live drama for more than 2 hours without a break, but people have to hold it in to enjoy a film? Senseless. One of the pleasures I get of watching movies at home with my wife is whenever nature calls, we press pause.
--Generally, if the film would be trimmed down, IT WOULD BE A BETTER FILM. Too often the long films are made by proven directors allowed to do whatever they want. But generally, some cuts would trim unneeded fat and make the film crisper, sharper, more effective. I enjoyed the 2005 King Kong remake (thanks to a vow of frugality, I will always be 6-9 months late on film commentary. I can rarely see paying to see a film in the theater when I'm already paying for Netflix and could see the movie in 6 months anyway. Plus, if films are "art" as some say, 6-9 months is certainly not too late to comment), but after 3 hours, I really wasn't as focused in as I probably should have been. And it would have been fairly easy to cut 20 minutes from that movie, and possible without hurting the overall feel (even helping it) to cut 30-40 minutes. Hemingway cut a big intro from The Sun Also Rises at Fitzgerald's suggestion; I've never heard it suggested that this unnecessary intro would have made it a better book. Am I thinking too much like a composition teacher, demanding the superfluous material be removed or edited? But most of the times I get done watching a movie, my comment is, "That was good, but it would have been very good if they cut ___ minutes."
*Before making a list of exceptions, please consider the meaning of "probably."