About 3 years ago I started martial arts training. One of the first things I had to unlearn was shallow chest breathing. Imagine that, 40yrs old and having to relearn how to breathe properly. It took me probably close to 6 months to adjust.
Yeah, I was surprised how long it took me to realize the error in my breathing. In hindsight, my athletic life before 21 could have benefited greatly from breathing "right".