I think this is also hard to answer, but just to describe my own experience, I started out with the No 6 series since that was the first one I tried that was doable. About a year and 1800 pages later I’d finished that one and noticed that a lot more things had become doable.
Among those were the e-book version of マリアビートル by Kotaro Isaka, which I bought and partly read, but then realized e-books make reading less interesting to me and abandoned it in favor of the 獣の奏者 series.
マリアビートル is a novel aimed at adults though, so I guess it should mean I had managed to clear the bar you described 
Not sure how much this helps, but hopefully it gives you some kind of indication.
Also note that my bar for “being able to read” isn’t full comprehension, but at least being able to explain what’s generally happening in the story at any given point. If a few sentences are incomprehensible every now and then, so be it.