How long does it take to form a habit? Probably longer than 21 days.
In the best-known study on the question, the average was 66 days, and the range ran from 18 to 254. Simple habits came sooner; harder ones took much longer.
Where the 21-day figure comes from
The "21 days to form a habit" line is usually traced back to Maxwell Maltz's 1960 book Psycho-Cybernetics. Maltz, a plastic surgeon, noticed that patients seemed to take at least about 21 days to get used to a change. Over time, "at least about 21 days" turned into "21 days", and the caveat got lost.
What the research found: 66 days on average
In 2010, a team at University College London asked 96 volunteers to pick one new daily habit and log it for 12 weeks, rating each day how automatic it felt. For the people whose data could be modelled, it took a median of 66 days for the habit to reach its peak of automaticity.
The spread was the striking part: from 18 days for some people to an estimated 254 days for others. In general, simpler actions became automatic sooner than more demanding ones.
Source: Lally, van Jaarsveld, Potts and Wardle, How are habits formed: Modelling habit formation in the real world, European Journal of Social Psychology, 2010.
What that means for you
Expect two to three months, not three weeks. If a habit still feels like effort after a month, that's normal, not a sign you're doing it wrong.
Start smaller than you think. Simpler actions became automatic sooner in the study, so make the first version easy and add to it later.
Tie it to something you already do. "After I make my morning coffee, I stretch for two minutes" gives the habit a clear cue.
Don't panic over a missed day. The same study found one missed day made little difference to the result. Here's what to do when it happens.
Keep going for the long stretch
Sixty-odd days is a long time to keep anything up, so it helps to see how far you've come rather than only today. Flaric, the iPhone habit tracker we make, shows your progress over weeks and months and gives you a few days off each month, so one rough day doesn't send you back to zero.