“한국말을 얼마 동안 공부했어요?” 은 무슨 뜻인가요?
I think it means “How long have you been studying Korean?” However, I’m not sure. Please help me. Thank you!

Question

Answer

As you guess, 한국말을 얼마 동안 공부했어요? means "How long have you been studying Korean?"

Strictly, it means "How long did you study Korean?"
한국말을 Korean
얼마동안 How long
공부했어요? Did you study
Because there is no present perfect or present perfect progressive tense in Korean, it is expressed as a past tense.