How long will it take to lose weight?

I have a goal to lose as much weight as I can over the summer. I have been exercising lightly and eating smaller portions and lost 20 lb but I still have 100 lbs to go. For a goal weight of 180 lbs (I have a wierd body frame, when I was 180 I was fit/muscular and still a size 14-16). When I start excercizing more heavily/changing my diet more radically, do you think its possible to lose a significant amount of weight in 3 months? Or should I aim for like 50lbs instead? (I'm a female, age 17, height is 5 foot 8-9 inches, current weight 300 lbs). Anyone have experience with this?