According to Google, short-form content is content shorter than 300 words. And as soon as your text goes beyond 800 words, Google sees it as long-form content.
However, this statement is not black and white. Your competition also plays a role here.
How? Easy, if no one in your industry writes articles with more than 200 words, than your article with 500 words will be characterized as long-form. On the contrary, in the industry of digital internet marketing, companies may write articles of 10,000 words. If you want to compete with them, you face a lot of writing and equivalent research to do.