Have to agree with Angela on this.
God has only led me to fast from water once, and it was like half of a day.
And I felt not led to fast further, because God said in my heart something like, see, this is how My Word is important for you and how much it is needed, and I made water so needful that you might understand this about the Word of God. I had that "lesson learned" feeling, filled a glass with water and drank it, and thanked God.
I've been led later to fast from food again, but never from water. It's vert important not to fast on our own without Spirit guidance. I've seen videos on youtube of a girl who has done two or three consecutive 40-day fasts and visibly wrecked her health. That, per example, is apparently not by God's guidance. God Himself will put on your heart to fast and instruct you exactly how.