The longest I've read fanfiction in is Naruto. I've read Naruto fics since Quotev was Quizazz, GoToQuiz was popular, and Quizilla was still around.
These are in no particular order and only how they pop up in my memory because there's a LOT. There's also a lot of anime in here, you've been warned.
Naruto (manga/anime), Bleach (manga/anime), Hetalia (anime), Lord of the Rings (including the books/movies and Silmarillion), Wolf's Rain (anime), JoJo's Bizarre Adventure (anime), Transformers (comics/movies/tv shows), My Hero Academia (anime), Castlevania (anime none of the games), DC Universe (and a ton of the other movies/shows but none of the live actions), Stardew Valley (it's a good game please don't judge), Legend of Zelda (also a good game), Pushing Daisies, Five Night's at Freddy's (also a good game), Pet Shop of Horrors (manga/anime), Harry Potter (movies and half the books), Ghibli Films, Megamind, ATLA and Legend of Korra, Demon Slayer, a sprinkle of JuJutsu Kaisen, X-Men, Legion (2010), almost all of the Predator movies (this was real recent since I watched the other movies when Prey came out and holy moly are they good), Rango (love me some 'what happened after the end' fics), Star Trek (old but only for Data and new), Wolf Children (because that movie ripped my fucking heart out), and Loki (2021 series).
This is all I can think of atm, but if you have some you're curious about and want to ask if I've watched/read from a certain fandom or if you'd like an elaboration on what I read in a specific fandom ask away!