Not that long ,almost around what Rubis suggested,yet there are many aspects to learning a language (French here in particular).Pronunciation in French is not very easy specially for you as a native English speaker, there are some vowels that might take you lots of practice to be able to pronounce them correctly.This part might take you a bit longer to master.
As for the basic vocabulary it won't take you long to get hold of it.
As a whole it will vary from person to person according to their own ability,the time they dedicate for learning and how intensive their effort is .