Your body needs time to get rid of fat, and trying to brute force it will lead to bad consequences. If you want to lose it, try intermittent fasting and work on the reasons you gained in the first place, don’t go for an extreme fast like this.
Do you mean no calories at all for that many days? Like is there a caloric limit like 600 a day or none at all? Or do you mean following a fasting cycle like 14:10 or something? Just trying to understand