It depends how I've acquired said film and how much I have invested in it.
If I've bought it on DVD, or paid for a ticket to see it at the cinema, I've invested a few bob and will probably wait it out and hope for the best even if it's not shaping up to be a good film.
If it's something I'm watching on TV or SKY or sent from Loveshite, that puts it into disposable 'take it or leave it' category. I will give it between 20 - 30 mins to interest me. If it hasn't done it in that time; it's out. I don't give one whether a purist would argue that I can't make an informed decision by watching only a part of it. Yes - I can. I've seen so many films that my instinct guides me and I'm fine with that. Life is short, I don't have time for crap, or for something that isn't doing it for me. I know that some of the stuff I reject is not poor quality, but it's not my scene.