1) I've never walked out of a movie and I'm determined never to do so for content-related reasons. I refuse to even take restroom breaks, instead forgoing any drinks for at least an hour beforehand.
Closest would be Eyes Wide Shut, but that was because the relative I brought with me was being a dick in insisting we leave. Other movies that made me want to leave: The Spirit was hideously obnoxious and unpleasant to sit through, Topsy-Turvy and Street Kings bored me to tears, and when I got roped into second-showings of Transformers, Spider-Man 3, and Avatar, I got really restless because they were so much worse on a repeat viewing.
Also, when I got prodded into seeing Transformers: Dark of the Moon and The Amazing Spider-Man, I convinced my companion to get tickets for Horrible Bosses and Safety Not Guaranteed, respectively, because I didn't want to be an economic accomplice of a social problem.
There are movies I only saw partially due to scratched DVDs that I've been meaning to watch. I haven't watched television since... before the year 2000, but I don't really count catching random scenes from that as any kind of commitment.
2) I'll stay for the credits of all but the worst movies.