It is possible, but only when you're learning a language which is closely related to the one, you speak fluently. If you're an Italian and you have great motivation, I think, to learn French, Spanish, Portugese (but not Romanian)in 6-12 months is possible. If you're a German, then to learn Dutch in 6 months is probably, possible, Swedish maybe in 8-12. 

As a Polish, I have never had problem with learning Ukrainian, Slovak, I think if I really needed these language, I could learn them completely in 6 months or so.

Frankly, I'm counting on the idea that it might be possible.

My history with foreign languages sounds a lot the Taiwanese woman Chris refers to in his discussion. I am also fantastic with grammar and I have always gotten A's in my courses, but no matter how hard I studied, I was never functional in any foreign language. I studied four: Spanish, French, German and Russian.

My mistake was that I thought if I could read and write, speaking and listening would just sort of magically happen. I now understand that all four skills are distinct and must be exercised independently. They will all reinforce each other, but excelling in any one skill will not compensate for weakness in another.

I started with reading and writing because those skills were most easily accessible to me as an independent learner and play to my personal strengths. Sadly, my listening and speaking skills are still as bad as they were in September, but I know I have some grammar and vocabulary in my pocket. That gives me the confidence to reach out to French speakers and to try speaking and to concentrate on listening.

My target is to be functional in French within a year. I don’t have to be able to discuss the intricacies of Descartes, but I want to be able to order some wine and cheese and make a joke or two with a server. Maybe it’s the American in me – we are eternal optimists, after all – but I believe this time it is going to work.

I think it depends on the environment you're surrounded. Living in a foreign country you'll probably learn any language within a year or maybe a little more, but if you don't live in a country that speak a language you're studying it becomes hard.

I defend the idea we need to be in contact with the language as much as we can.

Make some calculus, if you spend 3 hours of you day living your target language (which is a lot of time if you study, work, hang out...) how much will you need to live the equivalent of 6 months living in foreign country? 180 days x 8 hours = 1440 hours of target language, but living in your country practicing 3 hours a day you'll need 1440 hours / 3 hours = 480 days.

It depends too much of your effort, passion and motivation.