There is no point where you can say that, not if you want a career publishing fiction, and certainly not at the age of 18. If you want to write fiction to make money, you should be reading the classics and reading in your immediate genre as the books come out.

Reading widely and often is a good thing and, while you may see improvement just from writing, the benchmark by which you're measured as a writer is that other people think you're good. It doesn't matter whether you think you can get by not reading; you really need to be part of your own audience to know what they want out of your books and what they are buying now.

Just like a chef will never stop eating and trying new dishes and seeing how different flavours and techniques work together, a writer never stops reading and following their audience. You're effectively tasting other people's work so you know first-hand as a reader what appeals to you, how you read, how you get engaged by a book, why things considered poor style are that bad, and so on. In content, you understand whether your audience has moved on from certain subjects (e.g. the gonzo journalism of the 1970s probably would be too white-male-focused and misogynistic for today's market steeped in voices of colour and female experiences, and a writer trying that technique would be advised to make their female cast less passive and objectified if they really want to try that kind of thing now), whether you can get anyone interested in the world you set your fantasy epic in (while LOTR is still read as a classic right now, its white-bread Western European setting and lofty idealism is making way for grittier and more diverse worlds where there's more moral equivocation and bittersweet endings), and whether your readers are interested in yet another bundle of tropes you picked up from reading about a genre on Wikipedia, or whether you know what they want next because you're up to date in your reading.

That's really why you need to keep up and enjoy reading prose fiction if you want to be a prose fiction author. There aren't any shortcuts like films or video games because you're marrying story and tropes with the art of writing a prose fiction novel.

You need to keep reading.