Probably differs by genre for me.
I'll bail out of comedies fastest because if I'm not feeling it after ten minutes then I'm definitely not going to enjoy it.
I gave Bridesmaids five minutes or so and was hating it, so I fucked it off.
Action and horror I'll give slightly longer. Up to twenty minutes.
Dramas I'll probably give longer, depends innit.
What's the film you gave up on quickest?
Mama Mia for me. Switched off after two minutes. Later was forced to watch a further 18 minutes. Utter dogwank.