Jonathan Edwards gives one possible explanation for the infinite duration of hell:
The crime of one being despising and casting contempt on another, is proportionably more or less heinous, as he was under greater or less obligations to obey him. And therefore if there be any being that we are under infinite obligation to love, and honor, and obey, the contrary towards him must be infinitely faulty.
Our obligation to love, honor and obey any being is in proportion to his loveliness, honorableness, and authority…. But God is a being infinitely lovely, because he hath infinite excellency and beauty….
So sin against God, being a violation of infinite obligations, must be a crime infinitely heinous, and so deserving infinite punishment…. The eternity of the punishment of ungodly men renders it infinite…and therefore renders no more than proportionable to the heinousness of what they are guilty of.
(Quoted in Let the Nations Be Glad.)