Answer:
こんにちは英国政府のさんですが、私はそこにいなければなりません
This depends on your age and what language you are trying to learn really. Think about it this way, when you were born, how long did it take you to learn English (or your native tongue)? About five years right? This also depends on a bunch of other different factors like how much you practice, your determination, and the fluency of the language your speaking. So really there is no real definite answer because all people are different, but if I were to say, I'd say about five years...
An interpretation is a way of explaining something
