Only a very little Arabic and that's it?
So that's it? No more Arabic? I have friends who have been doing language with you for several years - and 90 days and the Arabic is done?
Well, it seems like the tree is long enough to give you a solid basis to the language (semitic languages has different concept non semitic speakers need to wrap their head around- not easy! let alone different set of letters!).
I think you might be mistaken what Duolingo is all about- not meant for fluency. If you feel you know the tree 100% maybe it's time to move on- audiobooks, books\newspapers, podcasts, TV shows, movies, etc. You can look into ArabicPod 101which I believe is free.
P.S - if you posting it now, after 90 days, know that it's a pretty new added course- that been 'pulled' for repairs at least once, so yes could be it.