On average you will lose about 1 pound a day. First you lose a lot of water weight, the the weight loss slows down a bit to 1 lb to 1/2 lb a day, maybe more if you are overweight.
So it would take like 100 days of fasting...
I'm in the same boat. I have like 120 lbs to lose. I am going to do it by fasting on and off, and eating mostly fruits and vegetables in between.