The 21 days myth
Maxwell Maltz, a plastic surgeon listed his observations in a book called psycho-cybernetics. The book was all about how his patients took exactly 21 days to adjust to their new body contours. Maxwell also observed that he needed exactly 21 days to automatically adjust to the new habits. Was Maxwell's observation on himself a classical confirmation bias? The single number of 21 days says it all.
The research gives a range of 18-252 days
The research conducted on 96 volunteers looked at when these volunteers perceived the habits as being done automatically. Now, the research had a answer which ran into a range of 18-252 days.
Finally, we have the statistics speaking and a range that is quite acceptable for everyone.
If we look at quote about Mark Twain on statistics
"There are three kinds of lies: lies, damned lies, and statistics."
The quote is often said that it is attributed wrongly Twain. But for the above durations of 21 days or 18-252 days, this statement of statistics holds good. An observational study and a sample of 96 volunteers cannot be generalized. The habit formation might take shorter time in some of us and a still longer time for others.