[hamstercat]

I can't comment on Cantonese, but I've heard that most people never truly learn Japanese unless they went to school in Japan. Apparently you can learn the basic in a year or two if you immerse, but learning the more complex stuff can take a decade or more.

[alphonsegaston]

This is the kind of mythical chauvinism that some Japanese people like to espouse, but it’s by no means true. Fluency is entirely possible as people like Seidensticker, Keene, and many others have proved.

[adrianN]

"Truly" learning any language probably can take a decade or more. I'd wager that many native speakers reach the level of "truly" understanding their native language. The Oxford English dictionary for example lists more than 150000 words in current use. If you want to learn them from scratch in just ten years you need to learn forty new words a day. That's not a sustainable rate. If you want to be able to read old texts (Shakespeare?) you probably need at least the 50k obsolete words that the dictionary also lists.