Exod 17:8-16 and Deut 25:17-19 relate the story of the Israelites doing battle with the nation of Amalek, and God saying he would remember the Amalekites and their sin of opposing His chosen people. We see from I Sam 15:2ff that God made good on that promise hundreds of years later; He ordered the Israelites to destroy Amalek for their previous transgression. Time did not forgive the Amalekites’ sin.
In Matt 5:23-24 Jesus uses an Old Testament scenario to illustrate His New Testament teaching that if we have sinned against someone, our first priority should be to go make that right with the offended party. Jesus puts no time limit on this important requirement. If we sinned against a brother a hundred years ago, we still need to go and ask for his forgiveness. It takes a humble heart.
In Matt 18:15-17 our Lord gives (preparatory) New Testament teaching that if our brother sins against us, our duty is to lovingly try to get the offending party to repent so He can be forgiven by God. There is no time limit put on this requirement either, yet I have known Christians who have stood strongly against the sins of other Christians at one point, but then over time they weaken and compromise, and start bidding Godspeed to those same erring Christians – even though there is no evidence of any repentance. If brethren sinned over 30 years ago, we still need to continue to rebuke them as we have opportunity (Gal 2:11-12, Eph 5:11), until they repent (Luke 17:3). And we need to warn other faithful Christians to do the same (Acts 20:31, Col 1:28). We have to stand strong until the day we die (I Cor 16:13, II Tim 4:16), not just until retirement age.
And if we have sinned against God, there is still no time limit. Others may forget about our sin, but the Amalek story referenced above (and many other passages) teach us God expects us to repent no matter how long it has been since we sinned against Him. Jesus’ ultimatum to repent (e.g., Luke 13:3) does not change with the passing of time. If we secretly told a lie, we are going to have to ask God to forgive us for it (I John 1:9) – or we will be lost (Rev 21:8). It matters not how long ago we told the lie. Couples in adulterous marriages (per Matt 19:9) are going to have to terminate those marriages, regardless of how long they have been in a “loving” relationship, and regardless of whether or not they have children together (Ezra 10:11,44). One act of fornication (I Cor 7:2) has to be repudiated, even if it occurred long ago in the days of our youth.
Conclusion: Do you know of sin that needs to be made right with man or God? No matter how long it has been, better take care of it before it is too late (Rev 14:13).