I am F/26, 5’3 and about 145 lbs.
If i had to estimate 20 days. Idling around ( doing the american minimal exercise ) i think people burn like 1500-1800* calories a day and a pound or fat is 3600 calories. Working out speeds it up. If you want to find out how much calories you burn a day. Fast for 4 days. Weigh yourself each morning then throw away the first day of fasting and take the average weight you lost of the three days. Multiply it by 3600. There you have your calorie burn rate.
*Different for every person.
You lose 1 ib per day during water fasting. However, tracking ibs during fasting is not the smart way to do it. Track body fat percentage. Tracking ibs is typically water weight and this can fluctuate massively during any diet.
Every human body is different, some lose more, some less. Do it the smart way and do your research.
Good luck.