I'm well past 2000 hours in Genshin Impact and I still have no clue when will it end.

Even taking the limited time events aside and asking someone to make an account right now and do everything that game has to offer to this date should take easily 600+ hours.

Skyrim on PS3. Luckily I didn't run into any of the catastrophic bugs that ended questlines like others did. Runner up probably Just Cause 2.

Death Stranding, every order on legends of legends.

I've hardly 100% anything but LEGO games.

In Ridge Racer Type 4 (PS1) I did collect all 321 cars and that's probably my biggest.
Or Angry Birds PC, with all the updates that had quite a lot to do.
Or Flatout: Head On (PSP), getting gold at everything was reasonable effort. But despite couple things, I don't feel that was nearly as tough as RRT4 or even Angry Birds.

oh, so many... I've got Gran Turismo 5 and Path of Exile platinum of hellish annoyance

Probs Witcher 3.

OG gotg titanfall also.

Forza Horizon 3 and Forza Motorsports 4 got all the achievements for both took over 400 hours for each game to get them all. Also got all the achievements for Assassin's Creed Odyssey recently took close to 200 hours.

Final Fantasy XV - platinum

Took me a while lol

I even played all the missions and side stuff that came out after I got the platinum as well.

GTA SA.

Precious few games I 100% because of length, difficulty, or engagement level.
I have 100% in the Sony Spider-Man games (minus DLC from the first game), and I think that even includes all upgrades to gadgets (that would perhaps be the only thing missing)
That's as big as it gets. Even in Persona 5 Royal I didn't get all the compendium finished.