How long do habits take to make or break?

For a long time I held the belief that habits take 21 days to make or break.
The theory is proposed by countless coaches and personal development trainers. So many, in fact, that it has become common knowledge (occasionally, we encounter a variation, such as the '30 day' habit too).
There's the rule that states, 'repeat a behaviour for 21 days and it will become so ingrained that it becomes habit'. This rule applies to making new habits as well as breaking old or bad ones. I read another version the other day: 'repeat a behaviour 21 times and it will become a habit'.
I've encountered this not only in coaching training but also in my study of sports psychology.

It is common knowledge ...

... actually, it is rubbish.

The idea seems to stem from Maxwell Maltz's excellent book: Psychocybernetics.
That book, written in 1960, predates most of what has been written about personal development. Indeed, much of what is written and taught today on personal development and self-help owes a great deal to this little book.
So what about this habit theory? The late Dr Maltz, a surgeon, had carried out research on amputees and phantom limb syndrome. He deduced that it takes roughly 3 weeks for the brain to become accustomed to the loss of a limb. This is how long the new neural pathways in the brain take to form. Ok, nothing wrong with that, but, there is a huge problem with applying the results of that research to determine habit formation.

As a hypnotherapist, I encounter people who want help with habits. They might want to stop a bad one, or one that they no longer want. Or perhaps they might want to create a new productive or positive habit.
So what is a habit? Well the Oxford Dictionary defines it as 'a settled or regular tendency or practice' -- no surprises there.
If someone comes to see me wanting to change or create a regular tendency or practice, I'll expect him or her to walk out of my door having done it. Why should they wait 21 days or 30 days?

An example: if you habitually purchased your fish from a local fishmonger and one day suffered violent food poisoning from a piece of cod bought there then I reckon there's a fair chance you'd change that habit overnight.
Likewise, if you habitually drove the same way to work each day and one day there was some newly installed traffic lights that made it your journey slower, again, you'd probably change overnight.
Similarly, if you've smoked for 20 years and you want to stop -- and I mean that you really want to stop -- then I'd expect you stop the moment that you declared yourself a non-smoker. And likewise, if you are habitually late for appointments, then you can change that habit from the very moment you decide to be punctual.

In fact, if there is anything you want to change about your behaviour, or your thinking, then just decide to change it. Yes, maybe you'll benefit from a little assistance, but you don't have to wait 3 or 4 weeks for it to happen.