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.
There will be more to the course in later iterations, but it will take time. If you're interested in how far a more developed course can go, the Hebrew one is pretty good. I'm pretty fluent, but it had vocabulary I didn't know (particularly in topics like sports and politics).