EXODUS; 13;15; And it came to pass,when Pharaoh would hardly let us go,that the Lord slew all the firstborn in the land of Egypt ,both the firstborn of man and the firstborn of beast; therefore I sacrifice to the Lord all that openeth the matrix, being males; but all the firstborn of my children I redeem. NUMBERS;3;13; Because all the firstborn are mine, for on the day that i smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel,both man and beast; mine shall they be.; I am the Lord..............Some people don't believe the 144,000 have been collected yet, but God started setting aside the firstborn male just after the Israelites settled in the land he promised them. 12,000 were collected from each tribe. how long it took , i don't know.