In all the Epistles—the books of the New Testament written to the Church—not one time is the Church told to fast. That doesn’t mean we should not. Mention is made of fasting, but no rules are laid down, nor is the Church even encouraged to fast. The reason is, fasting is to be done as the occasion arises. Fasting does not change God. He is the same before, during, and after you fast.
But fasting will change you. It will help you keep the flesh under. It will help you become more sensitive to the Spirit of God.
It is good to fast when things are pressing in upon you and you need to wait on God prayerfully. Or, the Lord may. speak to you and lead you to fast. If the Lord lays a fast on your heart, do it! He has spoken to me in this manner several times. The longest I have ever fasted, however, is three days.