I would say it depends on your first language but I'd like to say 2 months. Not fluent but you know the basics. This is also assuming you don't put effort into remembering and studying the language.

I agree with parisqueen, but learning French as an English speaker is relatively easy. The languages are alike. My mother tongue, Dutch, is considerably harder, and a language like Japanese is impossible to get right. To be fluent, even when the languages are alike can take years. It's taken me half my life to learn fluent English, and not to boast too much, but when I'm in the UK, people think I'm English.

I learned Swedish pretty fluently in about 3-6 months after moving from England. I studied grammar and such at home in addition to listening to real life conversations. It really depends how much effort you want to put in, but being in the environment forces you to learn at least a little.

Wow that's a really impressive time, Swedish is pretty hard too, at least in my opinion.

anywhere from 2 months to 2 years. Spanish to French, pretty easy... then to English... pretty easy but not as easy. Spanish to Italian, way easy... Italian to French easy. Those 4 to German... mildly difficult. German to Scandanavian, mildly easy and vice versa. Scandanavian to Russian, easier than English to Russian... way easier than Spanish or French to Russian. Any of those to Chinese(Mandarin or Cantonese)/Japanese/Korean? LOL UBER HARD. And then between those 3? Not easy. If you know Hebrew then most Arabic isn't so much of a stretch... about the same as Spanish to English unless you go into central Africa or Pakistan... then it melds. Indian... idk. I just... Idk.