After watching 15 minutes of [italic]The Aviator[/italic], I decided that it was really boring and was tired of Leonardo Di Caprio pretending that hew as cool and intelligent like Howard Hughes and switched to [italic]Brokeback Mountain[/italic]. (Even rewatching another movie is a whole lot better than watching any movie that Leo stars in.)
15 minutes is the maximum amount of time that I will watch a movie before I turn it off. I mean if the director is smart then he/she would know to make the beginning interesting to keep the audience hooked. The first 15 minutes is also a good indicator of how good the rest of the film is.