It depends. For some languages that are close to your mother tongue (dutch for german speakers, italian for french speakers) all you need is about 200 hours of work. Of course, if you're after chinese, arabic or russian, you'll probably need at least 2 years. Sure it takes time, but much less than in a language school, because you can study when you want and have time, restudy parts you don't understand and don't need to follow the heavy pace of the less clever in the classroom.