How long does it REALLY take to break a habit?

When I was starting out in sales the world's No. 1 sales trainer asserted that it took 21 days to break a habit. 

I always found that somewhat intriguing given that I had tried and failed several times to quit smoking (once gave up for 14 months) before FINALLY deciding to never again smoke.

Still, I never really questioned the above assertion.

It wasn't until I was a fair bit older and had been coaching others that I realised how simplistic and wrong that belief was and why, after losing weight and getting to their desired goal, over 90% of people eventually put the weight back on with additional bonus weight.

21 days is a good start but it takes a daily conscious decision to reinforce the desired behaviour long after the 21 day period....long after!

Interestingly, I received an email newsletter from the same Guru the other day. In the newsletter he again made the 21 day assertion. 

I emailed back straight away telling him how much I loved his work (his stuff literally changed my life over 30 years ago) but that I thought that his 21 day belief was simply not valid.

Within an hour he had emailed me back to tell me he agreed with me.

Not sure why he persists with it?

Maybe people don't want to hear the truth?