Great question! 21 days is approximately the time required for a new behavior pattern to be created (i.e. for something to feel normal). However, just because you have created a new pattern, does not mean that pattern is strong enough to sustain itself without will-power or focused effort, meaning the new habit could be easily broken.
Based on all the latest research in neuro-science and studies of high achievers, 66 days is the average length of time required for most people to hardwire a new behavior pattern to the point of “automaticity.” Automaticity is the point where it becomes harder NOT to do the habit than it is to do it. In other words, the point where will-power is typically no long required. Keep in mind that 66 days is an average based on extensive research, not a perfect magic number.