Depends. If they are studying a new foreign language abroad such as spanish they could pick it up quickly if they've been able to master one other foreign language previously such as English. They say if you are bilingual or multilingual it's easier for students to learn much faster than if someone who only new how to speak one language.

Between 5 days and four years, more or less. It varies.

when living in a foreign country, your proficiency in the local language depends on how much you interact with the locals. If, as many Chinese students do when studying in foreign universities, you stay in a group that only speaks you language, you are not going to improve your (English), or whatever language you are trying to improve on. Language learning is not passive. The learner has to put in the time. For some, it can take longer than others.

Haha, I love this question.  Do you mean English or another language?  I'll give you what I know about Chinese students learning English, I can't really comment on other languages:  

I get so many people telling me Chinese students are clearly the smartest in the world because they can do so many things, such as learning English in a year!  Mmmm.  And then they go on to tell me that that's proof of their obvious superiority because all foreigners come here and take years to learn Chinese.  What they usually don't mention is that one way or another they've actually been learning from a very early age (for English anyway, or in older generations Russian), and that language schooling has continued throughout their education.  So by the time they get to university age (when a lot go abroad)  those that were diligent students or just interested in languages will have actually been studying the language for well over a decade.  Do they go somewhere like the US and magically learn English in a year?  No.  They already know English and can read and write fairly well, have a big enough vocabulary, and a good understanding of grammar before they get out of the plane.  But it takes a year to practice listening and speaking before they can communicate without much trouble because Chinese education lacks in that area for the most part.  But as Sorrel said it also depends a lot on what they do in that year, and their aptitude.

I don't know, you're missing a noun.

Im learning Dutch for half an year, it is so difficult, still a long way to go.
Who has suggestion of the best way to learn it?

Shouldn't take no longer than 24 hours...if it takes longer than we all got a problem on our hands..

It depends on each individual. I have a friend who came to America without knowing a single word of English, and she finished the last year of high school, plus four years of her undergraduate degree, in English.

And I also have friends who have been studying English for more than 16 years and can't speak it worth a damn, but can write it very well. It all depends on the individual... your mileage may vary.

First generation, the one that immigrates: Never.

Second generation: the one born there or brought as a child: just a few years.

Third generation: may speak Chinese but not read or write it.

i don't know, I meet more Chinese at home since going China. If they've been here 5 years, their English is still poor and their attitudes not so great.

 I learned all the Chinese I need in one minute- tim bo dong.