After three years of high school German, I was fluent enough by the time I started college that I probably could've gone to Germany and had little difficulty navigating social interactions, but the old adage, "Use it or lose it," is very apropos when discussing learning (and retaining) any foreign language. To my credit, I've tried over the years to get back into it, but with mixed results. When I run across a phrase (like the one above) that I struggle with ("knallen" had been long forgotten), all I can say is thank the gods for Google Translate. (It's not perfect, I know, but it does help someone like me who has just enough knowledge to end up saying something really stupid if I relied solely on memory.)
When I worked at the hospital many years ago I had quite a few enjoyable (if somewhat broken) discussions with a nurse who was a native speaker. She encouraged me at every turn to improve my grammar and vocabulary, even going so far as to supply me with German print magazines. Sadly, she quit before I ever got back to a comfortable conversant level, and thereafter it all fell by the wayside.