How long should it take an average person to speak fluently a foreign language? (26)

Phoenix is right.
In Asia they call that "going to bed with the book".
And it's the best way to learn a language.

As to how long it will take to become "fluent" that depends on a lot of different things.

If the target language is similar to your native language, or if it is a simple language, you should be able to pick it up quickly and be fluent in a few years. But if the language is very different and difficult, true fluency can take many years.

Just as an example.

English speakers can usually speak pretty good Indonesian after just a year in the country. Pronunciation and grammar rules are easy and it's written in western characters.

But the same people would take many years to learn to speak fluent Japanese.

Although Japanese pronunciation is easy, its grammar is very different from English and is fairly complicated.
Japanese writing has no resemblance to western characters, and the social rules that govern language are especially difficult to learn.

One might learn to "get by" in Japan in a few months of hard study, but true "fluency" would take many years for most westerners, and there is almost no way a western adult could ever learn Japanese well enough to pass for a Japanese on the telephone.

It's difficult for Japanese to learn English too, but a little easier because the anglophone culture is more tolerant of mistakes and doesn't put as much pressure on the speaker to be "perfect".