Everything has exceptions. Breatherians don't eat at all and live for years and years. You can watch some YouTube videos on Breatherians.
I also have never experienced a true hunger. But to answer your question, it is not only when all fat is used up, also when all nutritional reserves are gone you should feel the true hunger. If you are still not clean you may not feel the true hunger. Listen to your body, you will know when to quit. I have had few fasts that had to end in a day or two because something did not feel right.