I was a Farsi linguist and learned it at 18. I spent a full year at the defense language institute in Monterey. We did 8 hours a day or so in an immersive environment with 10 or so hours a week of basic vocabulary memorization. My class's failure/drop out rate was about 35% - and we had to "pass" a very difficult test to even be considered.

Besides the different alphabet - which is a very serious impediment, don't underestimate it - the grammar is different and it sounds totally unfamiliar. Also, with Arabic, there is a sort of base Arabic and then many different dialects.

After all that, I left the language school with roughly a high school graduate's reading and listening/translating level. And while I had a good accent, I could speak at maybe a 6th grade level. But I was still doing a LOT of translating things in my head - which is obvious if you're trying to have a conversation with someone.

It took another 2 years or so of pretty constant use to feel really fluent. A lot of people talk about dreaming in that language as a marker, but there was almost a switch that got flipped that allowed me to read things as casually as I do in English, or be able to have or listen to a conversation without making it my primary focus.

At any rate, I fell in love with the language, the culture, and especially the food. I haven't used Farsi in 20 years but even now I dream in Farsi - or whatever weird Farsi vocabulary is still hanging around.