There's really no upper limit for me. If someone out there manages to make a 100 hour long FPS campaign that stays interesting, all the more power to them
I loved every minute of the 40 hours I spent on one playthrough of The Last of Us 2 for example, and also every minute of the near-200 hours I put into one playthrough of Trails of Cold Steel 4.

Conversely, I also felt more than satisfied with things like Uncharted 2 and 3's 8 to 10ish hour campaigns and recently, Metroid Dread's 10 hours.

So yeah, as long as the game stays interesting to me, it really doesn't matter. Though I will say I like my RPGs long.