Cutting 500 cals. a day IN THEORY will enable you to lose a pound a week (1 lb = 3,500 calories).
I think a safe estimate is that you will lose 1-2 lbs per week ON AVERAGE over the time you are on South Beach. I've been on for a year and a half and lost about 75, and I still have more than 30 pounds to go to get to goal.
There are many weeks I did not lose at all, even when following the plan properly. Exercise can make you gain lean muscle mass and sometimes you lose fat and gain muscle and the scale does not move downward when that is happening.
My best advice is not to get in a hurry. You did not get where you are in a hurry; nor will you take it off in a hurry. Rather, you want to safely lose 1-2 lbs per week on average to get to goal.
I am not trying to discourage you, I just don't want to see you set yourself up to expect to lose 6 pounds a week when in all likelihood, it won't happen that way. It might, but probably not. And I don't want you to get discouraged.
You have the entire rest of your life to get to goal. The health benefits of this way of life will be more immediate, though. My BP has lowered and I am off my meds, and my triglycerides are much lower, as well. So it's all a matter of taking it slow and steady, one day of the rest of your life at the time.
P. S. Vit D enriched milk is a good source of that vitamin.
So be sure you are getting in your dairy.