Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Repentance remains open to the son of Adam, if he accepts it, until one of three things happens: until the sun rises from its place of setting, or the Beast (of the earth) emerges, or Ya'juj and Ma'juj (Gog and Magog) come out."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8905