A Generation is 40 years in Bible. Here are some examples.
Numbers 32:13 (ESV) - And the Lord's anger was kindled against Israel, and he made them wander in the wilderness forty years, until all the generation that had done evil in the sight of the Lord was gone.
Deuteronomy 1:34-36 (ESV) - And the Lord heard your words and was angered, and he swore, ‘Not one of these men of this evil generation shall see the good land that I swore to give to your fathers, except Caleb the son of Jephunneh. He shall see it, and to him and to his children I will give the land on which he has trodden, because he has wholly followed the Lord!"
Psalms 95:10 (ESV) - For forty years I loathed that generation and said, “They are a people who go astray in their heart, and they have not known my ways.”
Hebrews 3:9-10 (ESV) - Today, if you hear his voice, do not harden your hearts as in the rebellion, on the day of testing in the wilderness,where your fathers put me to the test and saw my works for forty years. Therefore I was provoked with that generation, and said, ‘They always go astray in their heart; they have not known my ways."
The Israelites wandered 40 years in the wilderness (Ex. 16:35; Deut. 2:7), in which time an entire generation died out (Num. 14:33; 32:13).
Acts 13:36 (ESV) - For David, after he had served the purpose of God in his own generation. fell asleep and was laid with his fathers and saw corruption.
In 2 Samuel 5:4 (ESV)- "David was thirty years old when he began to reign, and he reigned forty years ."
We also know that Jerusalem was destroyed within the generation Jesus taught (AD 30-70). So all of the prophecies were fulfilled in that generation. Although this may look like a little off topic, still it is related to the end times in the generation of Jesus' disciples and clarifies the confusion of the reader about the generation (30-70 AD).
Jesus said that the sign of Son of Man will appear in the sky (Matthew 24). This was supposed to happen in the generation of his disciples (Matthew 24, Mark 13, Luke 21). During the fall of Jerusalem in 70 AD, chariots and troops of soldiers in their armor were seen running about among the clouds, and surrounding of cities. This extraordinary incident was recorded by Josephus and Tacitus who lived during that period. This is also recorded in Jewish History Document "Sepher Yosippon", Latin Document "Pseudo Hegesippus", and Historian Eusebius in his Ecclesiastical History.
Here are the details.
Josephus (Jewish Wars)
Jewish War 6:289 (220.127.116.119) Thus there was a star resembling a sword, which stood over the city, and a comet, that continued a whole year.
Jewish War 6:290 (18.104.22.1680) Thus also, before the Jews’ rebellion, and before those commotions which preceded the war, when the people were come in great crowds to the feast of unleavened bread, on the eighth day of the month Xanthicus [Nisan], and at the ninth hour of the night, so great a light shone round the altar and the holy house, that it appeared to be bright day time; which light lasted for half an hour.
Jewish War 6:291 (22.214.171.1241) This light seemed to be a good sign to the unskillful, but was so interpreted by the sacred scribes, as to portend those events that followed immediately upon it.
Jewish War 6:296 (126.96.36.1996) So these publicly declared, that this signal foreshowed the desolation that was coming upon them. Besides these, a few days after that feast, on the twenty-first day of the month Artemisius [Jyar],
Jewish War 6:297 (188.8.131.527) a certain prodigious and incredible phenomenon appeared; I suppose the account of it would seem to be a fable, were it not related by those that saw it,
Jewish War 6:298 (184.108.40.2068) and were not the events that followed it of so considerable a nature as to deserve such signals; for, before sunsetting, chariots and troops of soldiers in their armor were seen
Jewish War 6:299 (220.127.116.119) running about among the clouds, and surrounding of cities. Moreover at that feast which we call Pentecost, as the priests were going by night into the inner [court of the] temple, as their custom was, to perform their sacred ministrations, they said that, in the first place, they felt a quaking, and heard a great noise,
Jewish War 6:300 (18.104.22.1680) and after that they heard a sound as of a great multitude, saying, “Let us remove hence.”
Tacitus, Histories, Book 5
"Prodigies had occurred, which this nation, prone to superstition, but hating all religious rites, did not deem it lawful to expiate by offering and sacrifice. There had been seen hosts joining battle in the skies. the fiery gleam of arms, the temple illuminated by a sudden radiance from the clouds. The doors of
the inner shrine were suddenly thrown open, and a voice of more than mortal tone was heard to cry that the Gods were departing. At the same instant there was a mighty stir as of departure. Some few put a fearful meaning on these events, but in most there was a firm persuasion, that in the ancient records of their priests was contained a prediction of how at this very time the East was to grow powerful, and rulers, coming from Judaea, were to acquire universal empire."
Eusebius Ecclesiastical History, Book 3, Chapter 8, Section 4-6
And the eastern gate of the inner temple, which was of bronze and very massive, and which at evening was closed with difficulty by twenty men, and rested upon iron-bound beams, and had bars sunk deep in the ground, was seen at the sixth hour of the night to open of itself.
And not many days after the feast, on the twenty-first of the month Artemisium, (97) a certain marvelous vision was seen which passes belief. The prodigy might seem fabulous were it not related by those who saw it, and were not the calamities which followed deserving of such signs. For before the setting of the sun, chariots and armed troops were seen throughout the whole region in mid-air, wheeling through the clouds and encircling the cities .
And at the feast which is called Pentecost, when the priests entered the temple at night, as was their custom, to perform the services, they said that at first they perceived a movement and a noise, and afterward a voice as of a great multitude, saying, ‘Let us go hence.’
Sepher Yosippon" is a 10th century historical Jewish document written in Hebrew that mentions about the fall of Jerusalem in 70 AD. Sefer Josippon also mentions about the vision of soldiers and chariots in the sky which we read in Josephus' Jewish Wars, Tacitus's Histories, and Eusebius' Ecclesiastical History.
Sepher Yosippon (A Medieval History of Ancient Israel) translated from the Hebrew by Steven B. Bowman. Excerpts from Chapter 87 "Burning of the Temple"
"Moreover, in those days were seen chariots of fire and horsemen, a great force flying across the sky near to the ground coming against Jerusalem and all the land of Judah, all of them horses of fire and riders of fire. When the holiday of Shavu'oth came in those days, during the night the priests heard within the Temple something like the sound of men going and the sound of men marching in a multitude going into the Temple, and a terrible and mighty voice was heard speaking: "Let's go and leave this House."
This vision of the chariots and soldiers in the sky happened during fall of Jerusalem in 70 AD.
You may ask what does chariots and the soldiers in the sky have to do with the sign of Son of Man and Bible and also generation in the bible. Here are some examples.
Jeremiah 4:13 (KJV) – "Behold, he shall come up as clouds, and his chariots shall be as a whirlwind: his horses are swifter than eagles. Woe unto us! for we are spoiled."
Isaiah 66:15 (KJV) – "For, behold, the Lord will come with fire, and with his chariots like a whirlwind, to render his anger with fury, and his rebuke with flames of fire."
2 Kings 2:11 (KJV) - And it came to pass, as they still went on, and talked, that, behold, there appeared a chariot of fire, and horses of fire, and parted them both asunder; and Elijah went up by a whirlwind into heaven.
2 Kings 6:17 (KJV) - And Elisha prayed, and said, Lord, I pray thee, open his eyes, that he may see. And the Lord opened the eyes of the young man; and he saw: and, behold, the mountain was full of horses and chariots of fire round about Elisha.
Zachariah 6:1-6 (KJV) - "And I turned, and lifted up mine eyes, and looked, and, behold, there came four chariots out from between two mountains; and the mountains were mountains of brass. In the first chariot were red horses; and in the second chariot black horses; And in the third chariot white horses; and in the fourth chariot grisled and bay horses. Then I answered and said unto the angel that talked with me, What are these, my lord? And the angel answered and said unto me, These are the four spirits of the heavens, which go forth from standing before the Lord of all the earth. The black horses which are therein go forth into the north country; and the white go forth after them; and the grisled go forth toward the south country."
There are other verses in the Bible that involve God and his chariots.
This vision in the sky and the fall of Jerusalem (which includes the destruction of temple) happened between 30 AD - 70 AD (40 years). More informations are available here.