What I've heard is that for size 12 and up you have to lose 15 to 20 lbs to drop a size. With the smaller sizes (and I wouldn't know), the increment between sizes drops to 10 lbs.
Right now, I'm a pants size 16. I will need to lose 15 lbs to wear a size 14. My ultimate goal is a 12 which looks good on me, since I'm 5'7".