I think 35 main story, and say 50-60 for main and side quests with additional time for trophy hunting is a good pace for an open world game. I didn't mind at all that Elden Ring was longer, but that's my favourite game in my favourite genre, so not fair to compare it with anything really. I do know that Assassins Creed games overstay their welcome so I never finish them.
Been a while since I finished an open world game, usually because I tend to explore and do every side mission I can, so I get too powerful so the combat becomes boring. Or I just get tired of them and something else comes along before I finish them. Then when I go back to them, I've forgotten so much, that I start over. Rinse and repeat.
Only games in recent history that I can remember where that didn't happen were Witcher 3, Elden Ring and Ghost of Tsushima. I did start to feel overly powerful in Elden Ring, then I got to the winter area and realised that was when the "Dark Souls" really began. For Ghost, the gameplay was just fun. It's the only game I've ever gotten a platinum on. For Witcher, I did everything until Skellige, then I just gave up and did the main story.
I am excited for Starfield, but doubt I will ever finish it if it's 100+ hours of "stuff" like most Bethesda games.