I went to school in China till 2nd grade and use Cantonese at home (like at a very basic level, I don't have the lexicon to discuss "adult topics" like politics, science, philosophy, etc...)

But that's enough to understand 99% of the plot of Mandarin/Cantonese TV shows with zero subtitles. I mean, sometimes there's new vocab, but your can figure it out with the context. I could also mute the sounds and read chinese subtitles, and still understand it that way. Read and Listen is easy, Write would be the challenging part.

I actually don't know exactly how many characters/syllables I could understand lol. But clearly you don't need a lot, just grade-school level is good enough.

I don't think people actually remember characters especially nowadays. Its not like alphabetic languages whete you can sound it out, because the same chracter has like a different pronunciation in each different Chinese Variant (aka: "Dialect"). In the past, I read about that they used to use pen and paper to communicate because of "Dialect" differences.

I remember like the characters for numbers and my Chinese Name, but I can't write a basic sentence using a pen, even though I can type it using Pinyin or Jyutping (Cantonese Pinyin). Idk if my parents can still write after being in the US for over a decade.