Hello Ania, thanks for the question.
uczyć się means to learn, to study so your question is:
Jak długo uczysz się polskiego? How long do you study Polish?
You probably wanted to ask:
Jak długo uczysz polskiego? How long do you teach Polish?
I did work before as an English teacher so I have some experience in teaching but with Polish, I started teaching it around 9 months ago 😀