SW say you should expect to lose around 1-2lbs a week, but it really depends on the person and how much they have to lose in the first place. Someone with a lot to lose will lose quicker at first than those with a stone to lose, but then it should slow down to 1 or 2 lbs a week. I started at a size 16/18 and it took me just over 2 months to lose my first stone.
I really wouldn't give yourself a target like that, as it might have an effect on your motivation if you didn't reach that goal. Just stick to the plan, and celebrate your losses week by week.