As a number of people have said- it's a very individual thing. I have the misfortune of being a painfully slow language learner. Despite living in Japan and soaking in the language, despite having Japanese family, colleages and friends- it takes a long long time for most aspects of the language (vocabulary, grammer) to make some sort of sense, and then longer still before I can use them with any confidence in conversation. It's certainly not from lack of trying, but I have a terribly slow ear and that makes everything difficult.