A parable concerning a group of bandits held captive in the King's dungeon: They dug a tunnel, broke through, and escaped - but one of them stayed behind. When the warden came and saw the escape tunnel, and this man had remained, he began beating him with his truncheon. He [the warden] said to him: "Fool! There is a tunnel leading to freedom; how can you not hurry to save your life?!"
So too, the time will come when the Holy One, Blessed is He will say to the wicked "Repentance was laid before you, and you did not return?!" (Medrash, Kohelet Rabbah)

Rabbeinu Yonah learned from this (Sha'arei Teshuvah 1:2) the gravity of the punishment reserved for the sinner who delays in repenting from his sins:

His punishment grows heavy upon him with every day, for he knows that there will be wrath upon him and that he has a way to allay that wrath...that is, repentance, yet he stands in his rebelliousness..."