The "21 days" thing became popular from a 1960's self help book lol. The plastic surgeon that made that statement lacked scientific basis or study, it's basis is that it took his patients a "minimum of 21 days" to get used to their new face. However a bunch of self-help book writers started writing in their books that it took about 21 days as a result, and it took off.

The leading 2010 study on 96 people forming a new habit finds that it takes some only 18 days at the least, and others as much as 264 days. The average between participants in the study to form a new habit was 66 days, or about 2 months, for the new habit to actually feel automatic. It also depends on the habit itself.

Anyway, I've begun exercising (not daily) and brushing my teeth daily. It's going okay. Brushing every day for over a month now! Still feels a little forced. Exercising I've been doing 3 days a week for two weeks successfully. I think it's going to take me more than a month to feel like it's an everyday thing, rather than feeling like "I have to do this". But I'm hopeful and I'm gonna keep going!