In 1968, I went to see Ulysses. I had a seat behind an elderly couple. The man began to frown the moment the intro music started. As soon as they realized that Odysseus would not be making an appearance in the film, they got up and left.
We took the girls to all the 'Lord of the Rings' movies on three consecutive New Years Eves. An 8:00 start got us out at 11:40(ish) which gave us time to jump in the car, get home in time for a toast, and send everyone to bed.
Not too far into 'The Two Towers' (the second one) an older couple stood up, walked out, and left no doubt whatsoever that they wouldn't be back for episode three. I've never seen anyone make a display of leaving a movie before (not that you said you've done that) but it was very memorable.