What would happen when you are all done with a language?
Well if you mean when you finish your course here, then you can start another language, or find other sources for the languages you've completed here. Duo is awesome and I love it but it is not enough to get you fluent in a language. It's a good beginning. Anyways, you're never really done learning another language, so it should be something you love.
As you are on level 6, I would suggest you get further along here first. However, I can recommend some apps for you:
HelloTalk - chat with native speakers of your target language who are learning English.
HiNative - used solely for asking language questions, with a native of the language in question answering. So it's great you don't have to pester your HelloTalk friends with so many questions!
Meetup - both website and app. This is used for in person group meetings over shared interests, such as languages. If it is available in your city, maybe you can attend a few meetups? I usually attend a Spanish on Wednesday nights and Italian on Saturday mornings. I absolutely love it!
I just started an extensive online program ($$), and I will continue with my streak here, because I still learn things, and it's excellent review.
My time on Duolingo helped me skip the entire first level at the other online program, and start comfortably at their intermediate level.
Finishing the tree is just the very start of the study process, you can think of it like getting max level in a MMORPG. If you had only used mobile up until this point, then you will be a few months behind in your studies, since it is much easier than the website. Next you need to get to the point that you can constantly do time practices and never fail. Next you need to be able to earn a 5.0/5.0 on the Quiz constantly. You should throw on there doing a flash card program if you haven't done already, like TinyCards and make sure you know 100% of the words. That will get you 2000 known words, which isn't enough to communicate fully in the language. You will need to know all proper forms and tenses of those 2000 words. After you have done all that, then I would say you know the tree and Duolingo is not offering you anymore, than keeping you refreshed. At that time you need to move on and get books, schedule tutoring online, or meet natives to expand your studies.