After finishing this course, how much will I be able to understand?
After having gone through all this stuff and mastering every skill, how much am I going to understand from a Romanian version of young adult literature? Just asking. A response would be nice, you know.
I haven't finished the Romanian course so I can't speak to that one specifically, however, I've finished French, Spanish, and Catalan, and I would say reading young adult literature afterward is very possible. I would consider starting with a translation of a book that you're already familiar with so you don't get too bogged down in looking up every single word you aren't sure about.
I doubt you will, Duolingo mostly only teaches basics to a language and considering that the Romanian course is in Beta - it is even more unlikely that you will be able to understand a young adult book. You could use other guides such as Memrise etc. along with Duolingo which could help you progress though.
Native romanian speaker here. The biggest issue I see with Duolingo's romanian is the fact that it ONLY teaches the written language (used in articles, essays, documents, etc.).
In spoken language, when the subject is a pronoun, it is typically not said. Example: "Eu am udat florile" is always said as "Am udat florile" in spoken language, because it is clear that the speaker is doing (or was doing) the action from the 1st person singular form of the verb. In reality no language has the same spoken form as its written form, thus you may want to try to find sources where you can learn more of the spoken form, especially in case you're willing to come to Romania.
Also, I'm not sure what kind of book you're willing to try and read, but if it's an older one, you may find alot of terms that you have never heard of. They are called "arhaisme" (words that are no longer used) and "regionalisme" (words used only in certain parts of Romania) in romanian. You may also encounter a few expressions, I recommend studying expressions in particular.
All in all, only with Duolingo and no interaction with actual humans you probably won't be able to read complex literature, but if you do finish and learn the entire thing, you're gonna be good enough for easier books.