Earlier than most people think, and with much easier material than they choose. The mistake is not reading too soon. It is reading something four levels above you and concluding you are not ready.
Start once you have a few hundred words
You do not need to understand everything. You need to understand enough that the story pulls you forward, which for most people arrives within a few months. Reading at that point does something no course does: it shows you the same words again and again in different sentences, which is exactly how they stop being fragile.
The one-page test
Open any page and count the words you do not know. More than five or six in a paragraph and the book will exhaust you before it teaches you. Children’s books are not the answer — they are full of rare animals and invented words. Graded readers, translated books you already know, and news written for learners all work better.
Stop looking up every word
Looking up everything turns reading into decoding and kills the pace that makes it work. Look up a word only when it blocks the sentence, or when you have now seen it three times. Everything else you let go. It will come back, and the second or third encounter is when the meaning arrives for free.
Reread rather than push forward
Reading the same chapter twice is worth more than two new chapters. The second pass is almost effortless, and that fluency is the thing you are trying to build. A short book read three times teaches more than three books read once.
Pick something slightly too easy, read it quickly, and let the unknown words go past. That is the version of reading that actually makes you better.