That's a really hard question to answer. How fast you lose depends on a lot of factors: how old you are, how adamant you are to follow the plan exactly every day, how much weight you have to lose, your gender, how easy it is for you to lose weight in general.
But I can tell you that I've been doing this for 6 weeks now and have lost 19.6 pounds. That's 3.2 pounds per week on average. But, then again, I'm relatively young, I have a lot to lose, I follow the plan 100% every day, and I typically lose a lot quickly than is normal.
I suggest signing up on the Nutrisystem forums (which is free) on the official website and posting on the boards there. They're really helpful and not nearly as quiet as it is here, in relation to NS.
Speaking of which...
Week 6 weigh in: This week, I lost 1.6 pounds, for a total of 19.6 pounds lost in 6 weeks. I'm almost to 20 pounds!