The words 长 (cháng) and 久 (jiǔ) can both be used to express “long” or “length” in Mandarin Chinese. 长 can be used to talk about physical lengths in space and lengths of time, while 久 is exclusively for lengths of time.

Here's how to use 长 and 久 correctly.

长 (cháng): physical length in space, or length of time

长 is the more versatile of these two words. It can be used to describe both spatial and temporal lengths.

Using 长 to talk about spatial lengths

Exampes:

这篇文章不太长。

Zhè piān wénzhāng bù tài cháng.

This article is not very long.

祝您健康长寿。

Zhù nín jiànkāng chángshòu.

I wish you health and a long life.

长城和长江都很长。

Cháng Chéng hé Cháng Jiāng dōu hěn cháng.

The Great Wall and the Yangtze River are both very long.

Note that 长 can be used to describe the length of time-like things, such as films, songs or stories. For example:

这部电影太长了。

Zhè bù diànyǐng tài cháng le.

This film is too long.

流行歌一般都不长。

Liúxíng gē yībān dōu bù cháng.

Pop songs are usually not very long.

他给我们讲了一个很长的故事。

Tā gěi wǒmen jiǎng le yī gè hěn cháng de gùshì.

He told us a long story.

Time and space are metaphorically linked in most languages. Using 长 in this way describes the length that the film or story takes up in a visual image of time. 久 cannot be used in this way: it can only directly refer to time.

As always, it's best to avoid over-analysing the grammar and focus on getting more exposure to natural usage.

Using 长 with 时间 (shíjiān) to talk about lengths of time

As well as describing time-like things, 长 can also be combined with the word 时间 (shíjiān) to describe lengths of time explicitly.

Some examples:

寄到这里需要多长时间？

Jì dào zhèlǐ xūyào duō cháng shíjiān?

How long will it take to post here?

他当老师的时间不太长。

Tā dāng lǎoshī de shíjiān bù tài cháng.

He hasn't been a teacher for long.

处理这件事要很长时间。

Chǔlǐ zhè jiàn shì yào hěn cháng shíjiān.

It will take a long time to deal with this.

时间长了，我就无聊了。

Shíjiān cháng le, wǒ jiù wúliáo le.

After a long time, I got bored.

你在这家公司工作了多长时间了？

Nǐ zài zhè jiā gōngsī gōngzuò le duō cháng shíjiān le?

How long have you been working at this company?

你每天工作多长时间？

Nǐ měitiān gōngzuò duō cháng shíjiān?

How long do you work for each day?

Note that in these examples, 时间 is required for the sentence to be valid. 长 cannot directly describe lengths of time on its own.

久 (jiǔ): length of time

Like 长，the word 久 is used to talk about lengths of time. Unlike 长, 久 can only describe time and nothing else.

A classic example is this saying which has ended up with a literal borrowing in English:

好久不见！

Hǎojiǔ bùjiàn!

Long time no see!

Notice how 久 is about time by itself: intensifying it with 好 makes it “a long time”.

Some more examples of using 久 to talk about lengths of time:

你在这家公司工作了多久了？

Nǐ zài zhè jiā gōngsī gōngzuò le duōjiǔ le?

How long have you worked at this company?

那已经是很久以前的事了。

Nā yǐjīng shì hěn jiǔ yǐqián de shì le.

That's already a long time ago.

不久之后，他就回来了。

Bùjiǔ zhīhòu, tā jiù huílai le.

Not long after that, he returned.

太久了，我们不能再等。

Tài jiǔ le, wǒmen bùnéng zài děng.

It's been too long -- we can't wait any more.

你等了多久?

Nǐ děng le duōjiǔ?

How long did you wait?

你等了多久了?

Nǐ děng le duōjiǔ le?

How long have you been waiting?

Notice that 多久 and 多长时间 are usually interchangeable:

你等了多长时间?

Nǐ děng le duō cháng shíjiān?

How long did you wait?

你等了多长时间了?

Nǐ děng le duō cháng shíjiān le?

How long have you been waiting?

Using 久 with 时间

久 can also be used with 时间, often without changing the meaning from 久 alone. In those cases 时间 is probably added to balance the sentence or add emphasis.

Some examples:

我想知道这需要多久时间。

Wǒ xiǎng zhīdào zhè xūyào duōjiǔ shíjiān.

I'd like to know how long this will take.

我想知道这需要多久。

Wǒ xiǎng zhīdào zhè xūyào duōjiǔ.

I'd like to know how long this will take.

咱们已经花了很久时间了。

Zánmen yǐjīng huā le hěn jiǔ shíjiān le.

We've already spent a lot of time on this.

长久 (chángjiǔ): a long time

Finally, the two characters 长 and 久 can be combined into a single word: 长久 (chángjiǔ), meaning “a long time”.

An example:

希望我们能长久合作。

Xīwàng wǒmen néng chángjiǔ hézuò.

I hope that we can co-operate for a long time.