Well, hours is unrealistic, soldiers did fight for days, but they did it in shifts. The front line would rotate so that that the second line could replace the frontline to allow them to rest a bit. Intense fighting could realistically last only 15 minutes before a human needs to rest. The reason armies could fight for days is due to the fact formations were deep, lines were rotated in and out, and wounded soldiers were ferried to the back lines to be treated. Even then most fighting between armies concluded by sun-down due to visibility concerns, it's hard to tell friend from foe when you could barely see. (Though night battles are possible, they require a great deal more coordination to execute.)
Given that Pauliana was not wearing full armor, I would say she could fight an hour max, which already puts her at the top 1% of fighters in the entire world. I mean a full 12 round boxing match is 47 minutes and Pro boxers have 1-minute breaks in between rounds and you can visibly see fatigue accumulate over those rounds.
Anybody that says three hours is anywhere close to realistic has not ever watched a fight in their life.
But this is a manga it doesn't matter. Who cares if it's impossible, you just have to understand that you won't ever see something like this happen in real life and that's okay because people don't read comics for their adherence to science or common sense.