Finishing the Tree
At what level of fluency is one expected to have after finishing the tree and KNOWING the tree very well? like an A2 or B1 level?
Depends... For the reading side I would say b1 or maybe even b2, but that is only part of the levels, without being able to actually communicate with a native speaker you may still even be a1. Duolingo is not a replacement for language learning, just an amazing way to augment other tools.
I just started using lang-8.com and I recommend it highly. It's not going to solve my biggest problem, which is speaking/listening, but it gets you to keep a journal in your target language and it has quickly become apparent to me how much harder it is to express whatever arbitrary thoughts I have than it is to translate short set sentences, even when I have time to think about it and look up words. And most importantly, your entries get corrected (often very quickly) by native speakers. The community there seems nice, like Duolingo's. People helping each other learn languages are apparently the nicest people on the internet!
(By the way, I realize this is only related peripherally to your topic--I mention it because I have also finished my tree and I've been looking for supplemental ways of practicing!)
I read somewhere around here or in an article about duolingo that finishing the tree will reflect a vocabulary of about 3,000 words, which puts you in the middle of B1, according to this website: http://ddeubel.edublogs.org/2010/03/16/vocabulary-does-size-matter/ It took me longer to finish the French tree than I had imagined, because while I'm not as fluent as a native speaker, I'm fluent enough to read and carry on conversations in French. As you progress in the tree, you're learning good vocabulary and grammar, but you're also studying specific sentences, in my opinion.
Also, you never really get to finish the tree permanently, as they'll tinker with it from time to time. You might finish it, but it won't stay finished.
Reading and translating articles will help with reading fluency. Speaking is a skillset that is addressed in the tree, but is no substitute for practicing with real people. There are lots of places you can go for that- even online if you don't have any native speakers in your community. Second life has areas where people do language swaps.