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:
Note that 长 can be used to describe the length of time-like things, such as
films, songs or stories. For example:
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:
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:
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:
Notice that 多久 and 多长时间 are usually interchangeable:
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:
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: