which level can you reach once that you finish the duolingo course ? like an A1 or A2?
I think this depends strongly on the level of commitment/dedication and memory of the person completing the tree. Ideally, if you were to remember and be able to use everything taught to you throughout the tree, there's no reason you shouldn't be able to function at an overall B1/B2 level. The only caveat to that I'd say would be vocabulary. There's only so much Duo can do in that regard. If you dashed through the tree, you'd probably struggle to communicate at a solid A2
I haven't finished any courses but I'm nearly there with Danish, and I'd say if you were only using Duo, you'd be maybe between an A1 and A2? But if you worked on your vocabulary alongside Duo you'd probably have the grounding for a solid A2/B1.
(But I do tend to grossly underestimate my proficiency levels so this is most likely waaay off