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It depends heavily on how close the language you are immersed in is to your native language. A motivated English speaker could be conversational in German or French in a matter of weeks, but would require extra effort in Arabic or Chinese, especially if reading these languages is also necessary.
Another factor is how much you immerse yourself. If you make no effort to practice and try to do different things, you may end up plateauing at the "ordering at a restaurant" stage.
I learned Spanish in about 6 months while living among Mexicans. But you have to study it on your own time too.
Its takes a long time to adopt that language. i am learning fourth language which is Spanish and its been two years. I still cant communicate in Spanish.