Never. I stood up to walk out on WC FIELDS AND ME, but changed my mind and sat back down. I did watch a friend walk out (quite justifiably) on James Toback's EXPOSED, but not before walking up to the screen and throwing his box of popcorn at it. The entire audience applauded him. It was definitely the highlight of that afternoon. I also went to see (the original) FUN WITH DICK AND JANE with a group of co-workers, one of whom got up and left about five minutes into the movie, only stopping to loudly tell us that we needn't worry about him, he'd just sit in the lobby and wait. So we let him.