The longest I ever needed to defeat a boss was the final boss of Elden Ring DLC, which is the hardest boss From Software ever done to a point they needed to nerf him twice since the release
It took me 2 hours with my build
And around 5 hours with my BF build (he couldn't beat it so I did for him)
I don't think anything ever came close to that
That said, some maps in Fire Emblem can take much longer than that. Soren's paralogue in FE Engage took me 5 tries on Maddening, around 8 hours divided in multiple attemps
Camila's paralogue took me 4 tries, until I realized I need to warp-skip her map because all the reinforcements were madness, around 6 hours waste I think
Elincia's Gambit on Radiant Dawn (hard mode) and some maps in Fates: Conquest (Hard mode) were terrible too, I remember getting stucked two, sometimes three days even
But this is because I was purposefully looking for the higher difficult settings, so in this sense is my fault







