This is obviously just my opinion, but I think the only way it might be possible would be to be surrounded by native speakers, spend half the day speaking with them and the other half of the day studying vocabulary and grammar.. And since true fluency to me includes reading and writing, you'd need to find time to work on those as well. The other factor would be how close your target language is to others you already fluent in. For example, I'd bet that a Spanish speaker could learn Portuguese in three months with some hard work, but as only an English speaker I doubt I could do so. I guess it's always possible if you are exceptionally adept at learning languages, in which case you'd probably already know a few and would know the answer to the question.

Yes, why not? Obviously you should consider a Jovian month, where each day is more or less 9.93 times a terrestrial day, therefore you would get 908.6 terrestrial days.

Maybe,the one who can do that,we call him a genius.

I don't think so. You can learn the basics in three months but not the whole language.

It depends!! What you really want to do with the language. Are you travelling in the country where you want to use the language, a hobby or part of study etc. Every different type has different level of proficiency. Yes you might get to a level where you can understand some part of conversation but getting fluent is not possible. This is my personal opinion. I am living in New Zealand and learning english from last six month.