When I was in Hong Kong in '68, our guide had learned English by going to the movies, and his English was superb (he was also busy reading Mao's Little Red Book). Admittedly, Hong Kong was filled with people who spoke English, but I don't think you would need a decade. My exchange student became quite fluent within about six months.
If you can listen to English speakers, I think that would do the trick. And I envy you your bilingualism.