When can I start reading?

The hardest part about reading will be the sheer number of vocabulary and grammar you don’t know, no matter when you start. You could learn 2,000 words, then pick up something to read and find you’re looking up 75% of the words you encounter. You can learn all grammar listed in Bunpro as N5 and N4 level, and find you don’t know what’s going on in upwards of half of the sentences you’re reading.

My recommendation is to learn as much grammar as you can through at least half of N4, then start (as @TheVegpieMunch mentioned) brute force reading. Not everyone will be suited to using this method, but it’s worth at least trying out. The idea is that you’re looking up all the vocabulary and grammar you don’t know.

This method of reading, called intensive reading, is a long, grueling process in the beginning. But if you stick with it long enough, and learn common grammar (which may be in the N3, N2, or even N1 range), and as you keep learning more vocabulary, you’ll slowly begin to transition to the extensive reading phase. This is where you know enough grammar and vocabulary that you can infer the meaning of some of the unknown words.

Because everything you read will have its own set of vocabulary, you can’t really gauge based on number of known words when you’ll be able to read something. Some manga I can read through without looking up more than a few words a chapter. Other manga, I’m having to look up every third word.

The key point is that you are reading something that interests you. Something that you won’t get bored with. For me, this meant picking a manga that was the source of an anime I liked (although the manga was way above my level, and I could easily spend two hours to get through four panels).

From there, I joined the Absolute Beginner Book Club, and focused on learning more grammar.

The next step up was to take a manga of a series I knew well (having watched the anime subtitled in English, and having read the manga translated into English), and read that through in Japanese. It was a simple enough manga, and I knew in advance the context of everything and what was going on, so I didn’t have to worry about all the words I didn’t know. My main focus was to catch any grammar I didn’t recognize, and learn that.

That’s the path that worked for me personally to reach a point where I can “read”, but I still have to pick easier manga. Some manga are just too dense with unknown words (I probably only know about 3,000 to 3,500 words) that I have too many lookups to enjoy reading it.

For learning grammar, the biggest boost you’ll get will be from reading material that uses that grammar. The more you see the grammar, the more you’ll build up pattern recognition. In time, you’ll recognize and understand the grammar instantly, without thinking about it, and without routing through English to understand it. But it takes a good amount of reading and having to manually understand to properly build up that recognition. There won’t be a point of study and review where you can suddenly recognize the grammar. It’s all in actually reading to get used to seeing it.