It depends on how you define “learn an instrument.” If you mean “make a sound”, that generally happens on day 1. If you mean, “play a song” that depends on what song, and how well you want to play it. There are simple songs that can be played at the first lesson on most instruments. There are advanced songs that take professional musicians years to learn to play (and there is everything in between).
We do not work on “seasons” or “semesters” like sports, schools or other programs. Many Music Time students celebrate anniversaries of 5, 6, 7, 8+ years in lessons with us. There is always more to learn (or other instruments to try). We consider “success” in music to be simply the continuation of playing. There’s really no finish line.