Obviously it differs from person to person. Some people just seem to have an aptitude for learning languages, making it significantly easier for them to learn any given language. Most people, however, will always struggle at least a bit with foreign languages. But don't worry, if you really want to and need to speak fluently, you will be able to. Because the only situation in which you'd have to speak fluently is when you actually live there. And believe me, no matter one's aptitude, when you live in a place where it's spoken as the main language, you will pick it up way faster.
As for aptitude; just look at the foreign language you are best at. How long have you been studying to get to that level and would you describe yourself as fluent in that language? Keep in mind that it's easier to learn when you're young so most likely learning Chinese will be harder. (Harder than English for example, because most likely you've had about 5 or 6 years of English already. Which means you started when you were about 13-14. Taking into consideration that learning a language is easier when you're young, you should be able to get an idea of how much effort it's going to take.)