Where does Paul say that the 34th verse of the 31st chapter of Jeremiah is fulfilled? There are many things yet fulfilled. Like the thousand year reign of Christ as described in the revelation of John. And I am willing to say that the passage in question is likely during that time. Especially if you read the whole passage in context.
Heb 10:
1The law is only a shadow of the good things that are coming—not the realities themselves. For this reason it can never, by the same sacrifices repeated endlessly year after year, make perfect those who draw near to worship.
2Otherwise, would they not have stopped being offered? For the worshipers would have been cleansed once for all, and would no longer have felt guilty for their sins.
3But those sacrifices are an annual reminder of sins.
4It is impossible for the blood of bulls and goats to take away sins.
5Therefore, when Christ came into the world, he said:
“Sacrifice and offering you did not desire,
but a body you prepared for me;
6with burnt offerings and sin offerings
you were not pleased.
7Then I said, ‘Here I am—it is written about me in the scroll—
I have come to do your will, my God.’ ”
a
8First he said, “Sacrifices and offerings, burnt offerings and sin offerings you did not desire, nor were you pleased with them”—though they were offered in accordance with the law.
9Then he said, “Here I am, I have come to do your will.” He sets aside the first to establish the second.
10And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all.
11Day after day every priest stands and performs his religious duties; again and again he offers the same sacrifices, which can never take away sins.
12But when this priest had offered for all time one sacrifice for sins, he sat down at the right hand of God,
13and since that time he waits for his enemies to be made his footstool.
14For by one sacrifice he has made perfect forever those who are being made holy.
15The Holy Spirit also testifies to us about this. First he says:
16“This is the covenant I will make with them
after that time, says the Lord.
I will put my laws in their hearts,
and I will write them on their minds.” b
17Then he adds:
“Their sins and lawless acts
I will remember no more.”
c
18And where these have been forgiven, sacrifice for sin is no longer necessary.