How long should it take to get a girl into the sack?

You can't really call her a girlfriend if all you want is a bed buddy. A relationship is built over time with respect and an actual care about what each other likes/dislikes/wants/needs and the things that you agree on. You need to get to know each other, and trust comes before sex. If she's as decent as you imagine, you won't get her into bed because she'll see straight through your obvious low opinion of her, only wanting sex.