In Matthew 24, where Jesus talked about the end of the age and His return, He mentioned that the sign of the Son of Man will appear in heaven. 

Mat 24:30  Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory.

The sign of the Son of Man is the appearance of a “new sun” shining in the heavens as the sky recedes as a scroll when it is rolled up.  See ‘What is “the sign of the Son of Man” that appears in heaven, Mat 24:30?’.

But when will it appear?

There is a passage in the book of Hosea which appears to give an answer to this question.  Hosea 5 is a prophecy about God’s punishment on Israel and Judah.  Let’s look at the last two verses and the beginning of the next chapter.

Hos 5:14  For I will be like a lion to Ephraim, And like a young lion to the house of Judah. I, even I, will tear them and go away; I will take them away, and no one shall rescue.
Hos 5:15  I will return again to My place Till they [Israel and Judah] acknowledge their offense. Then they will seek My face; In their affliction they will earnestly seek Me.”

Hos 6:1  Come, and let us return to the LORD; For He has torn, but He will heal us; He has stricken, but He will bind us up.
Hos 6:2  After two days He will revive us; On the third day He will raise us up, That we may live in His sight.
Hos 6:3  Let us know, Let us pursue the knowledge of the LORD. His going forth is established as the morning; He will come to us like the rain, Like the latter and former rain to the earth.

This is an end time prophecy.  At the time of the end, Israel and Judah, that is the English-speaking peoples and the modern-day state of Israel (see “Who are Britain and the USA in Bible prophecy?”), will turn to God after they experience great punishment.  This has clearly not yet happened.  God says He will be like a lion, tearing them and taking them away, as a lion mauls and drags away its prey.

Hos 5:14  For I will be like a lion to Ephraim, And like a young lion to the house of Judah. I, even I, will tear them and go away; I will take them away, and no one shall rescue.

This punishment is also described by Jeremiah.

Jer 30:4  Now these are the words that the Lord spoke concerning Israel and Judah.
Jer 30:5  “For thus says the LORD: ‘We have heard a voice of trembling, Of fear, and not of peace.
Jer 30:6  Ask now, and see, Whether a man is ever in labor with child? So why do I see every man with his hands on his loins Like a woman in labor, And all faces turned pale?
Jer 30:7  Alas! For that day is great, So that none is like it; And it is the time of Jacob’s trouble, But he shall be saved out of it.

The trouble to come on Jacob (Israel and Judah) is so great there is none like it.  But this same language is also used speaking of the great tribulation at the time of the end.

Mat 24:21  For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be.

There cannot be two separate events each so great there is none like them.  They must therefore be the same event.

The great tribulation lasts about 3½ years.  It is the last 3½ years before Jesus Christ returns.  See “How long does the Great Tribulation last?”  So the prophecy in Hosea 5 about God’s punishment on Israel and Judah is referring to Jacob’s trouble, the great tribulation.  It reveals that at the first two years of the great tribulation is the time of God’s punishment against Israel and Judah for their sins.

Hos 6:2  After two days He will revive us; On the third day He will raise us up, That we may live in His sight.

The time period is defined in the terms of days, but each day in prophecy equals a year in fulfillment as explained in Numbers 14:34.

Num 14:34  According to the number of the days in which you spied out the land, forty days, for each day you shall bear your guilt one year, namely forty years, and you shall know My rejection.

After two years of punishment, Israel and Judah will repent.  What will trigger that repentance?  Could it be the sign of the Son of Man appearing in the heavens?  Certainly when they see it, they know that this is the hand of God.  If this is the case, then the sign of the Son of Man appears in the last 1½ years before Jesus Christ returns to the earth.

Conclusion

Based on the above discussion, the indication is that the sign of the Son of Man appears in the last 1½ years before Jesus Christ returns to the earth.