When learning Japanese, there are various yardsticks students will use to measure their progress. Are you able to converse normally with Japanese on everyday subjects? Can you "think" in Japanese without translating things to your native language first? Can you describe to the pizza delivery man how to get to your house without him realizing you're a foreigner? (That's not easy, trust me.) It's common for Japanese people to compliment gaijin on their Japanese ability, but serious students know that the real goal is to get to the point where they don't tell you nihongo ga jozu ("your Japanese is very good") but just shut and talk to you normally. The official measure of fluency is, of course, the Japanese Language Proficiency Test, the 5-level test that helps you increase your language skill year-by-year, with the highest level (N1) being required for entering a Japanese university. I recently discovered a level that's even higher than that, when I was watching the Game of Thrones series with my wife and she asked me to explain all the relationships in the series to her in Japanese...Baratheon, Lannister, Stark and Targaryen. I was ready to die just then.
Want a challenge? Try explaining Game of Thrones to someone in Japanese.