Well, there was indeed a lot of answers already, but considering I have some experience reading Japanese novels (81 over the past 12 months), I feel I can at least talk about what I did.
Before reading books, I had spent a lot of time reading manga (since N4), which got me over the habit of trying to translate things in English or my native language, except in cases where I really didn’t get it and needed to organize my thoughts.
My first few attempts were around the time I got the N3. As people have mentioned previously in the thread, I basically had to look up everything and gave up after a few pages. Those books ranged from 楽しい ムーミン 一家 (so technically children book) to 獣の奏者 (but the furigana version, so also a book aimed at kids). I managed to make a dent into 獣の奏者, but that was it.
I kept reading manga for the time since it was much easier (and thus much more rewarding).
Around N2, with a lot more grammar and vocabulary knowledge, I came back to 獣の奏者 and actually managed to read it until the end. It took me 3 months, but I started from the last part I remembered; actual total time (including my first attempt) would be close to 6 months. I thought “yep, I did a thing, back to manga and anki”
I only really made progress in terms of reading when I was close to N1 (I read my second book ever days before the JLPT test, that I failed by 3 points). At that level, it took me about 2 weeks to finish a book, so I just kept reading. Reading itself started to feel much more natural toward the middle of the first book and after 3 months (i.e. ~6 books) I was actually fine not understanding everything I read. I would only look something up if really I couldn’t get the point.
I kinda plateaued at that level for a long while. I did get the N1 6 months later, but still, past getting better at the reading skill, my level of understanding and my reading speed did not improve.
At that point, I started using WK for real, which helped a lot with filling some of the holes in my kanji knowledge. Despite my N1 certification, I think I only “knew” 1500 kanji, which was a hindrance. The main problem (for me) was that the rest was not frequent enough for me to pick them up from exposure alone (especially in the case where there’s no furigana and I don’t look up the reading).
The last push that really helped was to use FloFlo (now koohi.cafe). Basically an SRS with vocabulary list generated from specific books. I used it both as a prescient dictionary (it knew exactly what I wanted to look up next), a way to actually learn those words, and a way to gauge book difficulty (since it told me how many words I didn’t know yet in a given book). That helped both getting book recommendations and finding books that would not be too challenging. (NB: you obviously need to use the system for some time to teach it properly which words you already know).
Another way to look at it is based on my kanji and vocabulary knowledge (approximated from my old anki stats).
- Around N3 grammar, 600 kanji and 3000 words, I couldn’t read, it was too painful
- Around N2 grammar, 900 kanji and 5000 words, I could read by looking up almost everything; still painful
- Around N1 grammar, 1200 kanji and 8000 words, I was fine-ish looking things up
- Around 1500 kanji and 9000 words, I got to a point where I felt comfortable not looking things up if I got the gist (note that it might be because of my experience reading rather than the slight increase in numbers
- Around 2000 kanji (thank you WK) and 13000 words (thank you WK + FloFlo), for the first time I reach a point where I didn’t need to look up anything in a book chapter. Arguably, it was in a later book in a series, and I had learned the vocab of the first book on FloFlo, but still it felt good.
Later (I’m basically not using SRS anymore, but have read more than 100 novels since then), I basically do not come across words I don’t know in most light novels. I’m reading the 薬屋のひとりごと series right now, which is set in medieval China, and err, yeah, I do need to look up things, but not that many anyway, so it does not really break the reading flow.
So, I think the tl;dr has been said many time already (get more experience; get more grammar and vocab), I just thought my experience would help getting some actual numbers. Note, though, that I have seen people on those forums manage books way earlier than I did, so I guess it also depends on your pain tolerance 
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