As a critic, I often get the same two questions, virtually back to back:
Have you seen (fill in the blank)? How was it?
Inevitably, I wind up giving one of two answers, if not both:
It was too long.
I wish it had been funnier.
There are very few movies to which those two answers don't apply. Mostly recently, I'd point to Avatar as the rare exception of a movie that clocked in at more than 2½ hours that didn't feel long. And, a couple of decades back, I saw The Last Emperor, then close to three hours long, and thought, "It felt like it needed to be longer." Sure enough, when Bernardo Bertolucci put out a director's cut that was 4 hours long, it felt just right. And, oh yeah, The Lord of the Rings trilogy - those movies all felt about right, even when they were expanded for the director's cut.
By contrast, when I saw Baz Luhrmann's bloated Australia, I remember the sinking feeling when they got to the end of a climactic cattle drive. I looked at my watch - because it felt like I'd been sitting there for days - and realized there was more than an hour to go.
Comedies, as a rule, shouldn't be more than 90 minutes long - 100 minutes tops. When you extend a comedy beyond that, you're treading on thin ice. You'd better be headed some place completely different from where you started or you're in big trouble.
Unless, of course, the movie is Funny People, which ran out of jokes after 45 minutes, then decided to spend almost two more hours being an unfunny dramedy.
The most recent example of that kind of overstuffing is Nancy Meyers' It's Complicated, a potentially hilarious bedroom farce that gets dragged out to two hours with a side order of encounter group and self-esteem therapy. But then Meyers is a filmmaker who doesn't know when to trim her own material.