It should be "你学习中文多久了？”or "你学习中文多长时间了？”You should notice that in English, we usually use question words to start a question. However, in Chinese, we always insert the question words into the narrative sentences to ask a question.

你学习中文多久了？

I agree with what the lady said before me. I just add the pronounce for you. (你学习中文多久了？ni xue xi zhong wen duo jiu le?) (你学习中文多长时间了？ ni xue xi zhong wen duo chang shi jian le?) I am not sure what kind of soundmark you use when you learn Chhinese. Therefore, I just put the soundmark in Chinese way.

你学中文多久了？ ni1 xue2 zhong1 wen2 duo1 jiu3 le?

谢 谢