Learning a language is always also time managment. Most people do not understand that learning does not mean intensity, but regularity. For sure, if you learn every day eight hours you will do a better progress than every day 20 minutes. However, reality shows that most people do not learn eight hours a day, because they cannot. They have either not the discipline or the time. So, 20- 60 minutes a day, and I really mean every day, are enough to study well. You should therefore try to manage one or two hours a day for your study. I cannot answer the question, how you can do this, because you have to do by yourself.

or did you mean how much time one needs to learn it well?

 

I'd say 1 year of intensive learning to get to the C1 level (advanced). 

I've reached B2 (upper intermediate according to european level system) in 6 months of intensive learning with a teacher but also individually.

Learning German seems to be a lifelong process. :)

If you want to learn it by yourself you'll need a lot of self determination because it really is very challenging. But if you find a good teacher, you'll eventually overpass the grammar dificulties or the pronunciation ones more easily.

You should blend its learning with your daily activities as for ex. reading news online in German or watching German movies if you enjoy such activities. At a beginner level you could adjust a certain lesson for a day and stick to it. Just as Rudravarman said, being consequent is everything.

 

Need I mean How do I need the time to learn German well?