Rate of Weight Loss During a Prep/Extended Fat Loss Phase

"My question is what kind of weight loss should we expect to have weekly when you start getting sub 8% bodyfat, and how would you determine the needed length of your prep knowing that you're not going to be losing 1-2lbs per week deep into your prep. For example if you were 20lbs above your projected stage weight, I know it's not as simple as 20lbs /2lbs a week = 10 weeks. I'm planning on competing next spring in the teen division before turning 20 and this will help me a lot, thanks."