7 day DF followed by 14 day WF is best. You’ll lose about 40 pounds.
But you’ll gain some back during the refeed.
Most people can’t just do a 21 day fast though. It’s a skill that is developed through practice.
If you can even make 5 days on a DF and lose 20 pounds, you’ll be more skilled than 99.99 percent of the population that can’t even go w/o food for 24 hours.