What is the best way to learn and how long will that take?
Is is better to quickly go through a language if one is able to, or to slowly learn? If the latter is better, how long will it take to learn an entire course? i.e. Spanish
Best way to learn is by focusing on the weak points in your vocabulary of the language. Also lots of practice helps. If you have someone that speaks that language you want to learn (might be hard to find for german) it helps to talk to someone who would be able to correct errors in your speech as you talk.
I think if you go too slowly, you are forgetting the language as fast as you are learning it. My advice, for what it is worth, is to devote as much time as you can without getting burned out and go through the course and supplement it with German stories, tv, and movies. There is a VERY animated young lady on YouTube and has a long list of videos; "Learn German with Ania", that I have found very helpful.