This would vary according to the language and the particular person's strenghths, of course. I probably could have gotten to the where I could have translated Spanish literature into English for a living long before I would have ever been able to pass myself off as a native speaker. Chinese would be much harder to get good at translating.
Translate it from what ?
It is one thing to translate a type, grammatically correct, edited document. It is quite another to document real-time voice, handwriting, etc. Think how hard it can be to read someone else's handwriting in English. The more expertise one has in a language the easier the problems are to solve. As a nation we need to apply national resources to solve translation problems. All issues must be managed. There must be tranparency in all the agencies. There must be accountability. There are no excuses for failure within our agencies now.