I think such things depend on what exactly you are practicing. Chances are if you deliberately practice a language that long, you'll likely be overly fluent. That said, if your body is such that you cannot physically pronounce that 'r' correctly, you'll never speak without some sort of lisp or accent. Art and music are just as much luck as they are talent: Talent can very easily overtake practice. People that practice art learn techniques, but after some time, tend to become very good at emulating. Some people just get art, though, and intuitively know where to place things, where to shadow, what marks to leave out. When the talented practice, they create and manipulate and take whatever they've learned and they become more awesome. Alternatively, formal and deliberate practice in art actually kills some natural talent with overt rules. I've seen all things. Who actually makes it and is regarded as an expert by society? Whoever gets lucky. Music is similar: Some practice and become experts in a symphony, some practice and create symphonies, and others yet, regardless of practice, still struggle to keep on beat.

In any case, though, I do think any person spending so many deliberate (or even not so deliberate) hours on one subject winds up with a pretty amazing knowledge base and probably knowledge of quite a few branches of knowledge. Even if the original goal wasn't actually mastered, still worth it for knowledge.