I see your point Verecund. For instance if certain bosses were 3-10min~ but there were more waves of mobs & bosses that kept coming. The combined time of the bosses could basically be the same as fighting 1 very long boss. Toss into the mix that the waves boss could be random & his behaviors might change each time you repeat it (so you're never sure what to expect).
What i'm describing sounds a bit like Bastion (FF11) but with random bosses & more waves. Also it wouldn't end that fast either.