Base85 contains just about every printable ASCII character, so I’ll use that as a base. 8516 ~= 1031 -> extremely huge, but still feasible at least for state actors. 8520 ~= 1039 -> if I read Wolfram Alpha’s comparison correctly, that is more information than is believed to be contained in the DNA of all living creatures combined. That’s why I’d recommend >= 20 characters.
1st. There are no official ratings, only self-imposed industry standards.
2nd. A USK or PEGI label is not even needed. The developers or publishers only need to click a handful of checkboxes to self evaluate their game.
Both is mentioned in the article.