Before your body starts to burn stored fat your body must first deplete the liver’s glycogen stores to signal the bodies requirement for burning more energy and fat.
If you begin fasting to lose weight it can take up to 14 hours of fasting before your body begins to burn the stored fat and then you start losing a bit of weight.
To be able to lose a significant amount of weight when fasting it can take up to 6 months to a year to see a good drop in weight.
The safest amount of weight to lose is between 5 to 10 lbs of weight per month so if you start losing too much weight at a time it can cause health issues and be unhealthy.
Along with fasting you should also get out and walk, run, jog etc to get enough exercise which also helps you burn off calories and body fat.
So within 14 to 24 hours of fasting your body should begin burning stored body fat as long as you don't break the fast.