This is an interesting thread. Good question to have asked...
I'm somewhere in the middle; after being raised in L.A., I suddenly found myself in a regular classroom with hundreds of other Japanese kids (Nihon Jidousha Seibi Gakko)...complete with Japanese textbooks. It was sink or swim...
How many kanji can you learn? At the end of four months, I was able to read all mechanics-oriented textbooks without any problem. How many kanji was that? Who knows? But don't ask me to pick up a book on politics... I'd be lost.
But as someone said - if you don't use it, you lose it. Even my mother (who is Japanese and saw B-29s in the skies above Tokyo) cannot read Japanese now after immigrating to the US in 1960.
