It tells you precisely in the book how long each phase (there are 2) should be done.

However, I really wouldn't recommend it. I know you want to get down lower before you get to basic but I think that being at your current weight and maintaining and/or possibly losing when you get to basic would look much better than maybe losing quite a bit of weight on a Fat Fast then gaining most or all of it back when you got to basic.

Just my 2 cents.

Debbie