“And they said unto him, Why do the disciples of John fast often, and make prayers, and likewise the disciples of the Pharisees; but thine eat and drink? And he said unto them, Can ye make the children of the bride chamber fast, while the bridegroom is with them? But the days will come, when the bridegroom shall be taken away from them, and then shall they fast in those days.” - Luke 5:33-35 (Also see Matthew 9:15, Mark 2:20)
Jesus said that after he is taken away that his disciples shall fast. Are you his disciple? If you trust him for salvation, if you believe on him, if he is your Lord and Saviour then you are his disciple.
Notice we are not told when to fast, or how often, but we are told how.
“Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward. But thou, when thou fastest, anoint thine head, and wash thy face;
That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly.” - Matthew 6:16-18
When we are right with God we will fast sometimes, but when and how often is up to how the Holy Spirit leads us. But we must remember the words of Jesus “and then shall they fast.” Then is the period of time in which we now live.
But for what cause would we fast? In the Holy Bible fasting is done in mourning, to repent, to seek mercy from God, to dispel evil spirits, to stay the judgment of God, to humble ones self before the Lord, to beseech God for deliverance, or an answer, or for a solution to a problem, and for many other similar reasons.
See Ezra 8:23, Nehemiah 1:4, Daniel 9:3, Joel 2:12, Matthew 17: 18-21 (Mark 9:29), Acts 13:2-3, 1 Corinthians 7:5, Judges 20:26,1 Samuel 7:6, 1 Samuel 31:13, 2 Samuel 1:12, 2 Samuel 12:16-22, Esther 4:3, Psalm 35:13, Psalm 69:10, Isaiah 58:1-11.
“Speak unto all the people of the land, and to the priests, saying, When ye fasted and mourned in the fifth and seventh month, even those seventy years, did ye at all fast unto me, even to me? And when ye did eat, and when ye did drink, did not ye eat for yourselves, and drink for yourselves?” - Zechariah 7:5-6
“ And whatsoever ye do, do it heartily, as to the Lord, and not unto men;” -Colossians 3:23 (1 Corinthians 10:31, Colossians 3:17)
For more "How to fast" information see the links below.
http://www.ccci.org/training-and-growth/devotional-life/personal-guide-to-fasting/index.htm