oh! I got it. How many days until the fast starts?
I would say until your last meal/juice is digested/absorbed and totally burned. In the case of juices - not longer than 2 to 5 hrs. You have also around 6 hrs worth of energy stored as glucose in the liver, after which ketosis should start kicking in. All this, provided that you do not have "leftovers" in your bowels.