The Occasion for a Fast is Voluntary:
Fasting was looked upon as a very great virtue in the early church. In fact, they thought so highly of fasting that they inserted the term "fasting" into the Biblical text even though it wasn't in the original manuscripts (check various translations or margin notes for Matthew 17:21; Mark 9:29; Acts 10:30; 1 Corinthians 7:5)! This emphasis upon fasting also caused them to do the very thing the Pharisees had done, which was to prescribe certain set times for fasting: twice a week on Wednesday and Friday!
We need to be careful to avoid pitfalls of legalism like this. Surprisingly, a particular day for fasting was commanded in Scripture only once -- on the Day of Atonement (Leviticus 16). The fast on the Day of Atonement was connected with a deep mournful spirit in confessing sin. Now in the New Covenant, Jesus Christ has become our atonement offering, so we no longer even need to observe the Leviticus 16 Day of Atonement! In all the rest of the Bible there are no other Scriptures which command fasting at a specific time or on a specific occasion! None!
So when should a Christian fast? When he or she feels the Spirit of God leading them to fast. The occasion for fasting is a totally voluntary decision. Some of the specific times when people in the Bible fasted are listed in the next section. But basically we can say a Christian may decide to fast whenever there is a spiritual concern or struggle in his or her life. Of course, there may be times when those in authority over us proclaim a fast, as was done by King Saul (1 Samuel 14:24) or Jehoshaphat (2 Chronicles 20:3). But normally and ultimately that decision is solely between us and the Lord.
The Length of a Fast is Voluntary:
When we were looking at a "normal fast" (see above) we noted that a fast was usually for one day. In addition to the Day of Atonement (Leviticus 23:32) you can see examples of one day fasts in Judges 20:26; 1 Samuel 14:24; 2 Samuel 1:12; and 2 Samuel 3:35. The Jewish day was counted from sunset to sunset, so this meant that the fast would be broken (that is, food could be eaten) after sundown. However, some fasts were longer. The fast of Esther continued 3 days, both day and night. At the burial of Saul the fast was seven days (1 Samuel 31:13) and David also fasted seven days when his child was ill (2 Samuel 12:16-18). The longest fasts we find in the Bible are for forty days: Moses (3 times -- Deuteronomy 9:9,18; Exodus 34:28), Elijah (once -- 1 Kings 19:8), and Jesus (once -- Matthew 4:2). The Biblical principle here is that the length of time you fast is determined by your own desires and the occasion or purpose of the fast. The duration can be that which the individual or group feels led to set. There is a great deal of freedom in the Lord here. However, the more common practice of a "normal fast" appears to be one day.
Wealth is not just what you have, but what you are.