The length of a generation in the Bible varies depending on the context and the book in question. In some cases, a generation is said to be 40 years long, as in the story of the Israelites wandering in the wilderness for 40 years before entering the Promised Land.
In other cases, a generation is said to be 70 or 80 years long, as in Psalm 90:10, which says, "Our days may come to seventy years, or eighty, if our strength endures; yet the best of them are but trouble and sorrow, for they quickly pass, and we fly away." This suggests that a generation is between 70 and 80 years.