I completed the second year of high school German by spending a few summer days at the local library. The teacher lent me a second year book for the summer. I'd started my language requirement late, in my junior year, and wanted to make up time and get to a more interesting class. I'm pretty sure it was not much more than 10 partial days' effort, and maybe less. This was without a German speaker to practice with, other than a pen pal. (Well, Dad still remembered some from his school days and travels, but he didn't use it much and I did this on my own.)
However, I had just gone on a school trip to Germany early that summer. Just nine days -- not long enough to really pick much up German -- especially amidst a herd of English speaking contemporaries. But it was a great trip and so very motivating. (I've remained friends with the tour guide to this day.)