If our sins are punished by eternal separation from God, why did Jesus only have to suffer momentary separation?
That's a good question, and I think there's a pretty clear answer.
Another question would be, How can one man suffer when millions should've suffered? Same kind of issue. How does one suffering become the suffering of millions? The math doesn't work! How does suffering for 3 hours on a cross correspond to delivering people from eternity in hell? All those kinds of questions apply here.
The answer is that the degree of suffering, indignity, reproach, degradation, and fall that Jesus endured is not simply determined temporally. And it's not simply determined by the exquisiteness of the pain of a nail cutting through a nerve in your wrist.
It's determined by the difference between the glory that he had with the Father in heaven and the ignominy that he suffered, naked and hanging like a piece of meat as the Son of God on the cross. It's that distance that is the magnitude that provides the scope needed in his suffering to cover an eternity in hell and to cover the sins of millions of people.
The way to think about it is that we commit a greater indignity against God, not just in accord with how many sins we commit or how bad they are, but in accord with how great he is. Therefore our sins are infinitely great because they're against an infinite person and deserve an infinite punishment.
Christ, being an infinite person, became so low that that drop in suffering, that drop in indignity was such a huge drop—it was an infinite drop—that it suffices to cover the sins of millions and to cover the entire length of eternity that we deserve to be in hell.
He is a great Savior.