‘多久’ (duō jiǔ) is an interrogative adverb meaning ‘how long’ — used to ask about the duration of a period of time. It combines 多 (duō), meaning ‘how much/many’, and 久 (jiǔ), meaning ‘long (in time)’ or ‘for a long time’. Together, they form a fixed phrase that functions as a time question word, always appearing at the beginning or middle of a question before the verb or predicate.

This phrase is commonly used in everyday spoken and written Chinese to inquire about lengths of time — such as how long someone has lived somewhere, how long an activity lasted, or how long something will take. It does not stand alone as a noun; it must be part of a question structure and is never used in affirmative statements. Unlike ‘多长时间’ (duō cháng shíjiān), which is more formal and explicit, ‘多久’ is concise and conversational, making it especially frequent in informal dialogue and beginner-level interactions.