Eternal judgment is not unjust because it lasts longer than the crime committed (we don't even do that in human justice); but it is just because it is proportional to the crime:
And just as the punishment of the first death cuts men off from this present mortal city, so does the punishment of the second death cut men off from that future immortal city. For as the laws of this present city do not provide for the executed criminal’s return to it, so neither is he who is condemned to the second death recalled again to life everlasting.Chapter 12:
Really, the only reason hell seems overly harsh to us is because of how wicked we are in our natures:
The more enjoyment man found in God, the greater was his wickedness in abandoning Him; and he who destroyed in himself a good which might have been eternal, became worthy of eternal evil. Hence the whole mass of the human race is condemned; for he who at first gave entrance to sin has been punished with all his posterity who were in him as in a root, so that no one is exempt from this just and due punishment, unless delivered by mercy and undeserved grace; and the human race is so apportioned that in some is displayed the efficacy of merciful grace, in the rest the efficacy of just retribution.As things worked out, God's justice and mercy are both shown, which could not have been the case if all had been saved, or if all had been left damned. And because so many more remain damned than are saved, those of us who are saved by grace through faith should be all the more grateful to God for His mercy to us.
Some of the Platonists argue that "hell" is simply a place of purgation, where mankind are finally purified and made ready for heaven. Christians doctrine, on the other hand, teaches that hell is permanent, and that only in this life and prior to the last judgment is suffering purifying. Augustine mentions that there may be some who are headed for heaven but suffer after death. This is not, however, a full-fledged doctrine of purgatory, not least because he is clear that no one suffers after the last judgment, and in no way does suffering justify.
Indeed, this life itself is purgatorial. In this world our life is suffering and purifying. If there is suffering after death for believers, it is only for the very, very few who did not suffer in this life. (I assume that Augustine believes that all Christians need to have at least some extent of similar circumstances for the sake of sanctification, and those of us who have not truly suffered need to catch up with our martyred and persecuted brothers and sisters.) Those non-believers who do not suffer have even worse in store for them...