It’s hard to say. I’ve owned theremins off and on for a long time, but only in the last two years have I been using one at my gigs regularly. So, I’m finally able to hit notes and melodies, but that’s from just hacking it out really and not so much a practice routine to get there. I’m still a total beginner compared to tons of people on YouTube. I think I’ve mostly improved because my ear has improved. I do have a book by Carolina Eyck, but haven’t had time to start it yet. I’ve been too busy gigging and creating new songs and mixes to work on actually playing better.
Yes, I’m a guitar player primarily, but even then I still don’t like my playing. My goal is to practice 2 hours a day all year, unless I have a gig, then that counts lol. (mostly because I’m too worn out to play or practice more). I gigged yesterday and today, so I haven’t started the hardcore practicing yet.
Long story short (without getting too much in the thick of it), I never really believed I could succeed at anything, so practicing seemed futile. A whole lot of counseling later and now I’m starting a practice routine, for my own enjoyment. I do want my playing to improve, just to make hearing myself at gigs more interesting.
If you are more discipline than I am, you can probably progress to see musical results playing theremin in a few months.