As far as I know, after childhood the resonance frequency itself normally doesn't actually change very much. Even as you practice more often, your resonance frequency should stay more or less the same. However, what does change is how strong your body responds to breathing at that frequency (as well as in general), and that should improve over time. How rapidly that happens is a good question though, and I don't have any concrete answer to it. It's probably highly individual, and I don't know any studies that look at statistics of a large number of practicitioners (although that doesn't mean such studies don't exist).

While it's not a quantitative change, Dr. Leah Lagos mentions in her book on the topic (Heart, Breath, Mind) that most of the people she works with feel clear subjective changes after around four weeks of practicing twice a day for twenty minutes.