The 21 days is a very old scientific assumption of time span wherein a habit is formed.
So many research today has already debunked this theory. Some says its 66 days, some says a year, some says habits can be formed in an instant.
Whether how many days or months we spend to form a habit, if we are really serious to make good habits, we must aim to stick to it as long as we can.
Why count the time for a habit to be formed, when we will be sticking to it for the rest of our lives.
Theories may come and go but one things for sure, good things are meant to last for a lifetime.