In my experience, and I've done two 3 month intensive courses in Switzerland, and part time prior to that during stints in Germany, it is important to get a good teacher.
The best I had was a lady called Lotte at the school opposite Bhf Wiedikon, Audio Visuelles Sprachinstitute. I got the basic Goethe certificate.
Of course the problem also is where you socialise, if you don't use it, you wont improve.
But I heard of people here 30 years that don't speak it.
K