As a computer musician: enough to construct a scale / mode and stay in it. Enough to construct chords. But I’ve never put the effort into learning notation because I don’t play an instrument (properly), and I’m grumpy about how much cruft there is in music theory. Like, the basic unit is the semitone, which should be called the whole tone. And the naming of intervals just pisses me off. “A fifth? Oh, that’s seven semitones.” Ugh. So I’ve never really advanced at chord theory.
I did take an intro to music theory class in college, but I every-good-boy-does-fine’d my way through it and it didn’t really stick. I got a lot out of “How Music Really Works” by Wayne Chase, though. No notation.

I know a bit. I took music courses all through highshcool. Also did some free self-taught courses when learning guitar which taught me modes. I struggle with identifying chords in a key by ear, but note intervals are pretty easy.

For me theory hasn’t been all that helpful beyond a basic understanding and knowing what others are talking about, and I’ve encountered a lot of people who seem to get hung up on theory details in situations where I would just trust my ears.

jazz bassist > me > rock bassist

just kidding i’m a drummer

of pencils on a desk.

I learned roman numeral analysis in school, but over the past few years of playing jazz, that all gets thrown out the window, as it’s too unrestricted and impossible to analyze mid-flow anyway. It’s better to find the music you like, listen to it and play it over and over, and you will end up absorbing and repeating it.

Music theory is to music what grammar or linguistic theory is to language: not necessary to speak it, and would just slow down the flow of something that’s too complex to think through.

I taught myself to play the guitar and electric bass by learning the chords and reading tabulatures, I got good enough to play decent bass in a rock band. I also learned the basics of playing the piano. For the first ten years I had only a very basic understanding of the underlying theory and I never felt the need to learn more. I didn’t know the names of the scales and modes I could play and I just knew in which order chords worked or didn’t, I just couldn’t explain why. Nor I needed to do so.

Then I chose music pedagogy as one of my university minors and suddenly I had to learn the theory and reading/writing notation in just a few months - it was not easy but in the end it paid off big time. I finally understood why things worked the way they did and it opened up a world of new ideas for me.

I’m happy that I learned the theory after I had already learned to play an instrument. Knowing the theory backs me up, it does not constrain me. Few of my friends went through the formal music education and some - not all - are really bound by “the rules”, breaking them in any way is a big NO when we play together. One can play almost anything with the piano if you hand him the notes, but he’s totally unable to improvise or pick tunes by ear. Which is quite baffling to me.

And I’m not saying that the route I took to learn music is better, it just worked for me.

EDIT: Typos.