Which CEFR level (A2, B1 for example) are we roughly on when we finish all the exercises in German?
I would say A1/A2 among all the skills, with strong and weak spots: you should be able to pick up the sound of the language and understand the basic idea of a discourse within the limited scope allowed by the taught vocabulary, written text will be easier to understand than spoken language and writing will be (a lot) easier than speaking, but still rudimentary.
TL;DR: written > spoken; understading > expressing
Duolingo is very good in gamifying the experience, keeping you motivated while drilling stuff in your head, what it can't do is give you a lot of inputs and interactions that a communication tool like a language requires to be mastered.
I would "guesstimate" at least decent B1 in reading and listening (German is not hard to listen to, and if you remember all the words you learned here, then you should be fine on B1 level).
Writing... it depends - A2/B1, depending on how much you will remember about the grammar etc.
Speaking - Duolingo doesn't train you to speak much, so it would be A2, if you remember words and know what you want to say.