To me “too long” means the main story is repetitive and doesn’t make you want to keep playing to find out what happens next.
Most of the games I play don’t get finished so the fact I finished the main story of AC Valhalla would suggest it wasn’t too long for me. I certainly understand the criticism that the meat of the main story, becoming allied with the various regions, was overly repetitive though.
For me, the highest compliment for an RPG-like game is when, after finishing, I immediately want to play it again to see what happens when I make different choices.
In the case of AC Valhalla, I’m mildly curious what would happen if I chose a different jarl for Snotinghamscire and a different ealdorman for Lincolnshire (say), but I have no particular desire to play through another 80+ hours of fairly repetitive combat to find out.