After 4-5 days of fasting your energy levels drop off (and your concentration). Within 3 weeks you will recover fully, longer and your body will start to consume itself and shut down organs. i.e. female will first lose their 'cycle' then more and more organ functions.

I didn't feel hunger after the second day. After 3 days you kind of enjoy not having to defecate. About day 4-5 you start to have less energy.

That's been my experience (I've never gone beyond 3 weeks).

-Nim