多久 (duō jiǔ) means how long as a question about duration of time. 多 asks about extent or degree and 久 means a long time. Together they form a question word for time duration. It appears in questions about past duration (你学了多久, how long have you studied?), future duration (还要多久, how much longer?), and hypothetical duration (要多久才能完成, how long will it take to finish?). Compare 多长时间 (duō cháng shí jiān), which is more explicit but equivalent.