(Short answer: if you are doing the action daily, between 2 and 3 weeks. Give it a few extra days to make sure it is strong enough.)

When does something you merely have done repeatedly, become a habit?

My sense from the 3 experiments I have done on this so far (and from many failed “experiments” where I tried to make something a habit before starting to apply ideas from The Power of Less), is that usually between 2 and 3 weeks of doing something daily, it goes from just something I’ve done a bunch of times to something that is habitual. That is, there is a distinct difference in how the action feels to do that seems to occur between weeks 2 and 3.

This accords with what I’ve read. Some places say 3 weeks (= 21 days) to create a habit, others 1 month (= approx. 30 days). One difference in how long it may take to create a habit, however, is in whether you are doing the action every day, or several times a week. For example, if you are doing it every day, in 4 weeks you have done the action 28 times. If you are doing the action 4 times a week, in 4 weeks you have done the action 16 times. So it might be, if you are doing the action several times per week (or as a way to start the habit out more easy just do the action a few times a week and then increase to daily), that you need longer to create a habit, and so it is better to focus on it for a month (or perhaps a bit longer).

So far, I have found that there is a difference in how the action feels between 2-3 weeks (14-21 days) as noted above, and that at about 25 days of (relatively) successful actions, I feel like it is a strong enough habit that I can continue with another habit creation goal. With the current habit I am creating (music practise), I will see how things feel about day 22. If it feels like the habit is strong enough, I will move to my next habit creation goal.