Sometimes I hear claims that one can learn a language and be fluent in it in a matter of weeks or just a few months. Learning a language is an extremely difficult and complex task, even for those who have mastered several languages. That having been said, there are probably some individuals who are able to create neural pathways that are optimal for learning and are able to pick up a language in a very short period of time.
There is the well-documented case of Daniel Tammet, who apparently learned Icelandic in a week well enough to speak it on a talk show in Iceland (sidebar: Tammet's case was researched by Simon Baron-Cohen, a cousin of Sacha Baron-Cohen, the actor who played "Borat" and "Bruno"). Tammet had apparently learned several languages in a short period of time. There is also Christopher, who displayed linguistic savant properties as well and could speak numerous languages. Many such individuals are otherwise afflicted with developmental differences (autism, or some form of brain damage), and have their linguistic prowess as a "splinter skill,"
There are also some polyglots whose talent does not arise as a function of a developmental difference and are considered relatively normal in other ways. There is a culture of talented polyglots who have been profiled; many of them started learning languages at an early age, but it is not unknown for people to begin learning languages as adults. There have been some studies of what it takes to become a distinguished polyglot, and it appears that part of it is dedication and work, and part of it may be attributable to brains that are wired a certain way. But there don't appear to be a lot of answers to whether such brain wiring is inherently innate, or can be achieved through learning. Here is an interesting series of interviews with polyglots in which they talk about language learning.
There is also the matter of what constitutes a polyglot. Some who call themselves polyglots may be somewhat faking it by knowing "just enough to be dangerous." One would probably have to go through a testing regimen to determine exactly how proficient they are in a given language. There are probably a lot of people who have various levels of proficiency in several languages that might fall short of fluency.
Now, let's get back to learning strategies that claim high levels of proficiency in short periods of time. There is a book that I had studied from called "Polish in 4 Weeks: An Intensive Course in Basic Polish" (one of several texts that I have studied from, there is also a "Part 2" book for more advanced learners that I am currently going through), but I personally took a lot longer than four weeks to go through it.
The theory is that there are 28 lessons, and if you do one a day, you will have it down in a month. There may be learners out there that can do that, but I certainly am not one of them. I would venture a guess that the average learner would not be able to meet that one-month time standard. You have to consider that the average student who takes two years of a language in high school or college can ordinarily barely speak the language, even if they made very high grades, unless they were extraordinarily motivated to actually learn the language rather than learn for the tests, which is possibly a different skill. This isn't to say that the subject matter of the book is not good; actually of the numerous textbooks I have on Polish, I have to say that this is one of the better ones.
Enough for now, but I'd like to revisit this topic again at a later time.