When he was four, my oldest sat through the 150 minute cut of Superman in total silence. When the end credits rolled, he burst into tears. Why? Because it was over. I think some of the film went over his head, but it was pretty much a religious experience for him (he's superhero mad).

My youngest didn't seem bothered by long films either. Perhaps it is in their DNA. I remember watching Doctor Zhivago when I was nine (over three hours) and being utterly mesmerised by it. :)