I went through a form of "total immersion" while I was studying Spanish. I had taken one semester of Spanish classes at night, then went to Argentina for three weeks at the start of the second semester of classes.
The purpose of the trip was to visit relatives whom I had never met before and none of whom spoke English.
I stayed with relatives during those three weeks, and heard nothing but Spanish day and night. When I resumed my Spanish classes after returning home, the instructor said it was clear to her that I had learned a lot in those three weeks. One thing I learned was how to speak more or less without agonizing over what I was going to say. In class, I was always hesitant for fear of making a misitake; when someone says something to you at the dinner table or while you are in the car on your way somewhere, you don't have the luxury of mulling over your answer before speaking.
If I could accomplish a higher level of fluency in three weeks, I'm sure that almost anyone else could do the same. If you could stay longer, so much the better. The important thing, though, is to be in situations where you don't have the option of speaking English.