The scriptures often refer to lengths of time as a generation.
1 Chronicles 16:15 Be ye mindful always of his covenant; the word which he commanded to a thousand generations;
Deuteronomy 7:9 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;
Matthew 24:34 Verily I say unto you, This generation shall not pass, till all these things be fulfilled.
How many years is a generation?