How effective is ketosis

In terms of burning fat + will an overweight person lose a lot of weight on a water fast
how long should the fast last??

Anyone will lose a considerable amount of weight on a water fast, but i would highly recommend you avoid this type of diet. This can lead to hypoglycemia and ketoacidosis. It makes a lot more sense to simply reduce your diet and exercise more. This way you gain muscle as you reduce fat rather than just reducing both whilst having serious possible side effects.