Was reading this article
on the BBC news ...then followed the link to this one
which is a bit older. Makes an interesting read "The bottom line is: stay strong. 21 days is a myth; habit formation typically takes longer than that. The best estimate is 66 days, but it’s unwise to attempt to assign a number to this process. The duration of habit formation is likely to differ depending on who you are and what you are trying to do. As long as you continue doing your new healthy behaviour consistently in a given situation, a habit will form. But you will probably have to persevere beyond January 21st."