Exodus 20.5-6 reads: "Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me; And shewing mercy unto thousands of them that love me, and keep my commandments."
Often this is read like this: iniquity to the third and fourth generation, and mercy unto thousands of individuals of them that love God.
Deuteronomy 7.9 seems to offer commentary on the Exodus passage: "Know therefore that the LORD thy God, he is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations."
Exodus 20.5-6 seems to be a parallel text: "third and fourth generation" with "thousands [of generations]."
In other words, evil has a short shelf life -- 3 or 4 generations (traditionally seen as a period of 40 years for a generation) while God's mercy is enormously long and full: thousand and thousands of years of mercy.
I'm not speculating about the relationship this might have with the parousia, but suggesting that because of our own limitations, we often delimit God's mercy. Exodus 20 -- in the midst of the giving of the law -- tells us otherwise. Psalm 30.5 is likewise good to remember here: "For his anger endureth but a moment; in his favour is life: weeping may endure for a night, but joy cometh in the morning."