The word "多久" is formed through the conceptual combination of its two component characters: "多" (duō), which fundamentally means "many" or "much" and often introduces a question about degree or amount, and "久" (jiǔ), which means "a long time" or denotes duration. By fusing these ideas, the compound directly constructs the core meaning of "how much time?" or "what duration?", where "多" asks for the quantitative extent applied specifically to the temporal sense provided by "久".