It's not an hour, its 81mins.well while your watching a movie, I expect you are capable of deciding whether or not you have enjoyed the plot so far. So if you didn't like the storyline, the movie buildup is too long or it didn't catch your interest, you won't continue watching it.
Just like Jonah Hex, when you start watching the first 20 minutes (of an hour long film) you kind of see that there is no story there, no character background, nothing there. Much like that hole in Jonah's face....
Plus I am not the type of person that would just walk out of a cinema & leave my mates to watch the rest of the film. Also I have paid to see the film, it is not possible to make a valid judgement on whether the film is good or bad unless I have watched the whole film.