I recently did some freelance work involving Gary Keller's The ONE Thing, which means I got to read and digest the business self-help/success title. I found the following research that Keller shared about self-discipline and habit-forming to be very interesting, especially since it partially debunks the "21 days to a new habit" mindset.
Here's what Keller had to say:
The right discipline goes a long way, and habits are hard only in the beginning. Over time, the habit you’re after becomes easier to sustain. Habits require much less energy and effort to maintain than begin.
In 2009, researchers at the University College of London asked the question: How long does it take to establish a new habit? They were looking for the moment a new behavior becomes automatic or ingrained. The point of "automaticity" came when participants were 95 percent through the power curve and the effort to sustain it was about as low as it would get. ... The results suggest that it takes an average of 66 days; the full range was 18 to 254 days, but 66 represented a sweet spot—with easier behaviors taking fewer days on average and tough ones taking longer. Self-help circles tend to preach that it takes 21 days to make a change, but modern science doesn't back that up.
So, 66 days is more than triple 21 ... this means you need to give yourself a fighting chance when working on forming a new habit. And don't kick yourself if you miss a day—as Keller points out, discipline is just part of the equation. Keep working on the new habit you're trying to form, and give yourself the time you need to do it.