Learning a language is a process which is never complete. How fast one can learn a foreign language depends on many factors, such as one’s affinity, age, motivation, dedication, persistence, etc. Taking into account the fact that a child can only be considered an advanced level native speaker at the age of 12 (which, as you will probably agree, still doesn’t mean that it will understand every word of the language), we can reach the conclusion that the time needed to learn a foreign language can be measured in years.
True enough, it is possible to learn a foreign language sufficiently to be able to have basic communication in just a few lessons. But let us assume that “knowing” a language implies being able to read books, write, listen and speak at almost the speed of thought. In that case, we can again conclude that it must take years to learn a language.
As it happens, my nine-year-old daughter spent 2 months abroad in an English-speaking environment. On the one hand, I can now say that she can speak English, but on the other, as an English teacher, I know that her knowledge of syntax and vocabulary has to be even better before she can adequately convey various shades of meaning which are always present in communication. So, yes - she is fluent and accurate and no – she still hasn’t learnt the language.
It takes years, then, to learn a language and you have to be prepared for this. And anyway, it takes a lifetime to learn!