i have finished only 26 lessons, in which level i would be when i finish the all lessons? A2 or b1? excuse my poor English
It really depends on you. How do you study? Do you watch videos or movies in German? Do you try to read stuff in German on the internet or elsewhere? Do you look up the words you are learning on Duo and find out more about them? Do you read and take in all the tips for all the lessons that have them on Duo? Do you cheat (take a peek at the words or answers)? It comes down to what you put into it. My understanding, from reading comments on this subject, is that to get to B1 you need to know how to talk some. So do you practice talking to anyone in person or on an app or some other way?
So to answer to this question; I would say that if you work very hard and do extracurricular (in this case, other than Duo lessons) activities. You could get to a B1 but it is not guaranteed if all you do is try and fly through the lessons.
Hardly A1 if you do only Duolingo and you find German hard. You could be A2 or B1 if you do lots of other work (see slamRN's post) and you find German easier than other foreign languages. Duolingo is great for vocabulary but you will have to study Grammar, Speaking and Writing in order to reach higher levels than just A1.