多长时间(duō cháng shí jiān) - how long (duration/time) in Mandarin

多长时间 (duō cháng shíjiān) asks how long something takes, stacking 多长 (duō cháng, "how long, of what length") in front of 时间 (shíjiān, "time"). Where 多长 (duō cháng) on its own asks about physical length, adding 时间 (shíjiān) turns the question toward duration.

It usually sits at the end of the clause, after the verb it is questioning: 你学中文多长时间了 (nǐ xué Zhōngwén duō cháng shíjiān le), "how long have you been studying Chinese?", with the completed-action 了 (le) closing the sentence to show the studying is still ongoing up to now.

English speakers sometimes reach for the shorter 多久 (duō jiǔ) instead, which asks exactly the same thing and is common in casual speech; 多长时间 (duō cháng shíjiān) is the fuller, slightly more formal version and is the one written out in textbooks and exam papers.