I have an easy answer to this one

CNN has an article asking "how long is too long for a movie?"

That's easy. A movie is too long when you notice your ass hurting from sitting in a movie theater seat. For some movies--"Miami Vice" comes to mind--that's about half an hour into it. For others, it never happens. They pretty much allude to this in the article; if the story is tight and holds your interest, you just don't notice if the movie is entering its third hour.

The story is as long as it is--great is great, regardless of length. Bad is bad, regardless of length.