What is the final level of Duolingo compared to the levels: A1-C2
It depends on the language you are learning and how similar it is to your language. Duolingo does not have a direct correlation with CEFR. Duolingo teaches mostly grammar and vocabulary.
So in some aspects of the language you maybe B1 and in others not even A1. In German if you only do Duolingo you will have most of the grammar for B1, but not totally, you will need to learn other topics, in fact you will not have even the whole grammar for A1. In listening comprehension you will be around A1 or less. In general if you only do Duolingo you may pass A1 or A2, some people claim B1, but I think that is not common. If you do Duo in addition to listening programs and more grammar and you try to talk a little and write, you can reach B1. The CEFR tests require preparation in addition to learning the topics.
I think this really depends on how you are approaching Duolingo. I am currently on level 11 which is not very high, but I can easily read academic literature and novels in German, maybe needing to use context clues or the dictionary for one or two words a page. I'm reading very complex documents from the 19th century in the old script for a current research paper. But I'm stuck working my way through these basic levels - not sure why? Part of it is format and intent of the program. I find the general advice that completing all the levels of Duolingo would only get you to A1 hard to fathom, but maybe if you only learn the skills and vocabulary narrowly presented and you are doing nothing else?