The basic expressions:
多久 (duō jiǔ)
It’s used to ask about the duration of time for both future and past events.
e.g.
A: 下午要开一个会。(Xiàwǔ yào kāi yīgè huì.) There’s a meeting this afternoon.
B：开多久？(Kāi duō jiǔ?) How long will it last?
A：不一定呢。(Bù yīdìng ne.) It’s not certain.
A：你愿意去上海出差吗？(Nǐ yuànyì qù Shànghǎi chūchāi ma?) Would you like to go on a business trip to Shanghai?
B：去多久？(Qù duō jiǔ?) For how long?
A：两个星期。(Liǎng gè xīngqī.) Two weeks.
多长 (duō cháng)
This is also common to be used in specific contexts to ask about physical length or the duration of time, often in more casual or spoken language.
e.g.
A：这是我们国家最长的桥。(Zhè shì wǒmen guójiā zuì cháng de qiáo.) This is the longest bridge in our country.
B：多长？(Duō cháng?) How long?
A: 165公里。(Yībǎiliùshíwǔ gōnglǐ.) 165 kilometers.
A: 我想买这个床。(Wǒ xiǎng mǎi zhège chuáng.) I want to buy this bed.
B：多长？(Duō cháng?) How long?
A: 两米。(Liǎng mǐ.) Two meters.
The uses in sentences
…… + 多久？
When 多久 is used in sentence to indicate the duration of time, we often put it at the end of it.
e.g.
这个会议要开多久？(Zhège huìyì yào kāi duō jiǔ?) How long will this meeting take?
你计划学习中文多久？(Nǐ jìhuà xuéxí Zhōngwén duō jiǔ?) How long do you plan to study Chinese?
…… + 多久了？ (duō jiǔ le)
It’s used to inquire about the length of time since something started or happened.
e.g.
你们在这里等了多久了？(Nǐmen zài zhèlǐ děng le duō jiǔ le?) How long have you been waiting here?
你在这个城市生活多久了？(Nǐ zài zhège chéngshì shēnghuó duō jiǔ le?) How long have you been living in this city?
…… + 多长时间？ (duō cháng shíjiān)
Literally “how much time,” used similarly to “多久” but can sound slightly more formal.
e.g.
你们的旅行会持续多长时间？(Nǐmen de lǚxíng huì chíxù duō cháng shíjiān?) How long will your trip last?
这台电脑你用了多长时间了？(Zhè tái diànnǎo nǐ yòng le duō cháng shíjiān le?) How long have you been using this computer?
…… + 多长？ (duō cháng)
When 多长 is used in sentence to indicate the length, we often put it at the end of it.
e.g.
这条河有多长？(Zhè tiáo hé yǒu duō cháng?) How long is this river?
你家的窗帘多长？(Nǐ jiā de chuānglián duō cháng?) How long are your curtains?
