I'm taking a C1 exam in July after showing up in Germany in October basically unable to say anything. So I could say 10 months is what it takes... but it's just more complicated than that.
I'm a native English speaker, and decided I wanted to do Grad School in Germany. So I took a year of German 101 at my local community college (~5/hours week), then sold all my shit, enrolled in an intensive language program abroad, and moved to Germany in October of last year. Couldn't speak worth crap when I first got here. Those community college classes pretty much only gave me an understanding of the grammar, but speaking and hearing were completely unpracticed.
But now I live in an apartment with three Germans my age and have started dating Germans and making German friends and other international friends with whom I speak German. And I've also been going to a Sprachkurs every day. 7 months ago I was A1 and now I'm on track to get a C1 certificate in two months.
But this isn't possible for everybody. I have studied foreign languages immersively before, so I've had time to figure out how I learn best. It also depends on how much time you devote to it and the quality of your course. 5hours/week in community college in the States when I was also working full time? I didn't learn very much, basically started from scratch upon arrival here. But 20hours/week while living in Germany?? That's when you really start to see the progress taking off and it's really crazy to read things I wrote even a month ago - the difference is palpable.
My advice to you is to focus on grammar while you are in the States. Read and write when you have the time, but don't stress about speaking and hearing comprehension to much - those two areas just come so much faster when you are living among native speakers. But grammar is SOOOOO important in German, and that is something you can learn from a distance. It will facilitate the rest of the puzzle falling together when you finally do arrive in Germany - and that's when you will really begin learning to speak and understand.
The other thing is, that when you get to Germany, just be really stubborn about speaking German. So many Germans speak really good English, and if it is easier for the two of you to communicate in English, they are going to want to speak English. It's sort of an uphill battle until you get over the hump where, all of a sudden, you speak and understand well enough that it is actually easier for Germans to speak German with you than English. THAT is a wonderful moment and you will learn and be exposed to so much more once you get there. But until then you can expect that Germans will automatically speak English with you when they hear you struggling... and you should be prepared to try your hardest to just answer back in German anyways
Also: I agree that Langsam Gesprochene Nachrichten are the bomb (I listen everyday!), but probably overwhelming until you're around B2.