(Exodus 1:6) 6 Eventually Joseph died, and also all his brothers and all that generation.
(Numbers 32:13) 13 So Jehovah’s anger blazed against Israel and he made them wander about in the wilderness forty years, until all the generation that was doing evil in the eyes of Jehovah came to their end.
(Deuteronomy 2:14) 14 And the days that we walked from Kaʹdesh-barʹne·a until we crossed the torrent valley of Zeʹred were thirty-eight years, until all the generation of the men of war had come to their end from the midst of the camp, just as Jehovah had sworn to them.
(Joel 1:2, 3) 2 “Hear this, YOU older men, and give ear, all YOU inhabitants of the land. Has this occurred in YOUR days, or even in the days of YOUR forefathers? 3 Concerning it give an account to YOUR own sons, and YOUR sons to their sons, and their sons to the following generation. Again, here, from you to your sons is a generation.
The following scriptures deal with Jesus concerning the generation that he kept talking about so much, that they were wicked:(Matthew 16:4) 4 A wicked and adulterous generation keeps on seeking for a sign, but no sign will be given it except the sign of Joʹnah.” With that he went away, leaving them behind.(Matthew 17:17) 17 In reply Jesus said: “O faithless and twisted generation, how long must I continue with YOU? How long must I put up with YOU? Bring him here to me.” (Matthew 24:34, 35) 34 Truly I say to YOU that this generation will by no means pass away until all these things occur. 35 Heaven and earth will pass away, but my words will by no means pass away.
So how long was this generation of Jesus time? Depending on where you start it at, it's about 40 years. The destruction of Jerusalem was in 70CE. So from when Jesus was speaking these things, say in 30CE to 70CE is 40 years.
Why does the Bible keep telling us that a generation is between 30-40 years, again and again. The reason is simple. It's because a generation is really that long, about 40 years, not 100 plus years. No where in the Bible can you get that using scriptures in their context, no where.