I've heard of Benny Lewis. It seems to me that his definition of fluency is the ability to speak a language conversationally. He doesn't claim that one must know the syntax, morphology, phonology, etc to know a language. If one had to know those things to know a language, then most people don't really even know their own language; they just speak it.
I think someone can achieve conversational fluency in 3 months. At least enough to be understood. For one to actually be able to discuss any topic in that language, including the grammar itself, one will have to take much longer though. Most second language learners will never achieve the fluency of a native speaker anyway.
Benny Lewis and other language hackers seem to me to try to provide an easy way out. Learning a language is hard for most people and most people do not have the time, energy, or motivation to learn one. By giving them an easy way out, one can feel satisfied by their achievement of learning a language. They haven't really learned a language though. They have only learned enough to be understood in simply conversations with sympathetic native speakers who are willing to diligently attempt to understand what they are saying.
The best way to learn a language is to understand the grammar, have a large mental dictionary, and be aware of regional dialects, idioms, slangs, and so on. It's far more difficult, but it is the only way to completely learn a language.