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...
The term "RSVP" comes from the French expression répondez s'il vous plaît, meaning "please respond." If RSVP is written on an invitation, it means the host has requested that the guest respond to say if they plan to attend the party.
