This het was hatched on 4/14, and this picture was taken just today, 5-21. She is over 5". From your description I would say your gecko is probably about a month to six weeks old, but growth rates are strongly affeced by heredity, temperature, and food intake. Low temps and lack of food can slow growth a lot. But you said she is well fed so I am assuming healthy growth rate.
Does she look about like this, in the way her pattern is breaking up/spots coming in (assuming she has a pattern)? Post a pic if you have one.
