How many hours of practice do we need to learn a foreign language?
All other things equal, things like effort and starting level, the answer depends on three things:
First, it depends on the phonological distance between your native and your target language. The greater the distance between the two languages in terms of phonemes, rhythm and overall melody, the longer it will take.
Secondly, it depends on the method you are using – the more efficient the method, the less time it will take.
Lastly on the level of second language mastery you are trying to achieve.