Conventional wisdom has been that it takes 21 days to make or break a habit. This "wisdom" is based on a few observations. The idea caught on, fueled by popularity and probably wishful thinking. Decide for yourself.
First, Dr. Maxwell Maltz observed amputees and found that it took (on average) 21 days for them to stop feeling phantom sensations in the missing limb. It took another 21 days to develop a new habit. This was huge in 1960, when his book Psycho-Cybernetics was published. Problems:
1. This was 1960. Over half a century ago, folks. I'd like to think we've learned a few things since then.
2. This wasn't even lab research. It was casual observations, based on physical healing from major trauma. And the subjects weren't "regular" people - they were recovering from ... yes, again, major trauma.
3. There wasn't any significant follow-up with the few subjects. Were they still maintaining their new habits years later? We'll never know.
This doesn't have much to do with your habits, either. Unless you're a chemist.
Second, a lot of lab research has found that rats and other critters are able to develop certain habits based on drug trials. Most of the habits formed at about the 21-day mark. Problems:
1. Clinical trials on animals are not the same as behavioral habits (sans drugs) of humans. We may as well say, well, it only takes once for a monkey to form a heroin addiction, so you should be able to form a new habit by doing something once. See the lack of logic?
2. Lots of these trials are 4 weeks long. So how impressive is it that by the third week, the animals acclimate?
3. Again with the follow-up. Since plenty of behaviors could be side-effects of drugs, it stands to reason that when the drugs stopped, at least some of the behaviors did, too.
It's time to toss this "21-day wisdom" out the window, because all it does is put unrealistic pressure on people.
Recent research (Lally, 2010) studied actual people doing actual behaviors with the intention of learning about creating new habits. Her study lasted 12 weeks. She found that the average amount of time it takes for people to form a habit is 66 days.
But that's not the whole story!
It depends on the behavior, first of all. Obviously some are simpler than others. People who chose to create an exercise habit took 1.5 times longer to make it a habit, compared to people who chose to make eating fruit at lunch a new habit. Most people, regardless of behavior, hit a plateau at some point. So even if you've jump-started a habit into high gear by day 21, it's entirely possible that on day 22, you're not going to be one day better at getting that new habit more strongly rooted.
Also, it depends on the timeline. If people missed a day once in a while, it didn't have any real effect. So it's probably not worth beating yourself up if you don't do your yoga or drink your water or whatever once or twice a week. That doesn't mean you can skip it for three days in a row! So maybe you've been good at getting up and jogging every morning for 21 days, and it's sort of a habit, but your car is in the shop and you're carpooling, so you have to leave for work earlier and you don't have time to jog. You get your car back on day 25. Habit destroyed.
What's this mean for you?
If you want to make a change, you have to allow realistic time for it. For small, simple changes to become habit, 21 days may turn out to be adequate. If you want a life-changing habit, prepare to invest at least a couple months before you think about slacking off.
Oh, and, if you're thinking about this kind of change, keep in mind that the first half the year is GONE at the end of the week. How are you doing on all those New Year's Resolutions? If you haven't achieved them, or you aren't at least halfway there, maybe it's time to think about a coach. Let's get some habits in place!
Lally, Phillippa, van Jaarsveld, H., Potts, H., & Wardle, J. (2010). How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998-1009.