It wasn't a movie theater but pretty much the only thing I've ever walked out on was a theatrical production of Joyce's Ulysses (in Russian) that had gone on for at least four hours already and threatened to last another two more. I actually didn't intend to leave at the four hour mark as I thought it had (finally!) ended at the end of Act 2 but it was just the brief break before act 3 but didn't have the heart to re-enter the auditorium. I have never been so bored and so relieved in my life.
I think walking out of a movie is kinda shitty frankly, but hey you do you.
If there's ever a film I wanted to walk out of, it's The Secret Lives of Dentists, which is an absolute ass melodrama from 2002 ish that has very vague shades of Fight Club starring of all people Dennis Leary and other actors you don't care about any more. It's such a trash film that dwells on a bunch of profoundly unpleasant people who have problems that aren't even problems that it still makes me angry. I loathe it and so should you (don't actually see this film).