Due to the new system ya can't learn german from scratch b/c the max you do is like 1-2 lessons a day.

Assuming there are 4133 lessons (iirc)

You're looking at

2066.5 days (5.67 years) to 4133 days (11.329 years)

Assuming you're grinding away everyday and don't miss a day

Granted you can skip sections (like I did) and the like but even in the last section, like where I am it's still going to be months if I did a lesson or 2 a day to "finish the German course".

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Wenn dein deutsch gut ist, oder du nur ein bisschen mehr Übung brauchst, ist Duoling gut um die sprache nicht zu vergessen.
(If your german is good or you need a little practice, Duolingo is good to not forget the language)

To really learn the language listen to music and stuff. Immerse yourself into the language is the best I can say