Kai, it is also hard for a kid to learn his native language to adult level in a short time.

Hello Kai. It depends from many factors:

-How much do you study(really the amount of time)

- How similar is this language compared to the others you know

- Are you living in a country where this language is spoken?

-What are your motivation to learning it? (a study showed that people who like also the culture or the type of native speakers learn faster tha language than people who learn it for other reasons, let say work)

In my case I learned Italian in 3 years and didn't have any problems in reading literature in this language. But I am living in the country, so it is a total immersion. I've studied also German for 3 years, but the results are not the same. 

Master a language is a very high level. Considering that people take university courses full time for another language, they also do not master their chosen language by the end of their degree.

Speak fluently is different and a lower level.