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.

Subsidiary question:

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.