Christian Fasting - How long should I fast?
There is not a set length that makes Christian fasting acceptable. However, there are some general tips that may help you decide on a duration that will work well for you.
In Bible times, fasts were generally one day in length. Judges 20:26 says, "Then all the sons of Israel and all the people went up and came to Bethel and wept; thus they remained there before the LORD and fasted that day until evening. And they offered burnt offerings and peace offerings before the LORD."
Occasionally, fasts in Bible times were three days (Esther 4:16) or even seven days (1 Samuel 31:13). And on three occasions, fasts lasted 40 days: Moses receiving the 10 Commandments (Exodus 34:28), Elijah encountering God (1 Kings 19:8), and Jesus being tempted in the wilderness (Matthew 4). Since these were unusual circumstances, it is normally not recommended for a person to fast for 40 days without medical supervision.