It would be my opinion this has not been fulfilled yet, and will not be till sin is no more.

Jer 31:34 "And they will no longer teach each one his neighbor and each one his brother, saying, ‘Know Jehovah!’ for they will all know me, from the least to the greatest of them,” declares Jehovah. “For I will forgive their error, and I will no longer remember their sin.”

Heb 8:11,12 "And they will no longer teach each one his fellow citizen and each one his brother, saying: “Know Jehovah!” For they will all know me, from the least to the greatest of them. For I will be merciful toward their unrighteous deeds, and I will no longer call their sins to mind.’”

IMO, these scriptures say this will occur when there is no longer sin in the world...

 

Most definitely...

It could be argued that when the chapter speaks of bringing his people back, this is the time of ingathering, the rebuilding of Jerusalem, that occurs before the New World, however the gist of the chapter speaks of the conditions in that New World.

As for the debate of when the call to rebuild comes, it's not really a subject to debate, for I can think of more then a handful of differing positions people take on that subject. Regardless, I hold that there are multiple fulfillment of the 69 weeks, and the final is literally 69 weeks before the New World, and brings in the final fulfillment of Jeremiah 31.

So the culmination of all this is that, I understand the wonderful blessings of Jer 31 as being fulfilled when Jehovah brings in his kingdom to this world.

 

Jer 31 is quoted by Hebrews;

Jer 31:31-34 "Look! The days are coming,” declares Jehovah, “when I will make with the house of Israel and with the house of Judah a new covenant. It will not be like the covenant that I made with their forefathers on the day I took hold of their hand to lead them out of the land of Egypt, ‘my covenant that they broke, although I was their true master,’ declares Jehovah.” “For this is the covenant that I will make with the house of Israel after those days,” declares Jehovah. “I will put my law within them, and in their heart I will write it. And I will become their God, and they will become my people.” “And they will no longer teach each one his neighbor and each one his brother, saying, ‘Know Jehovah!’ for they will all know me, from the least to the greatest of them,” declares Jehovah. “For I will forgive their error, and I will no longer remember their sin.”

Heb 8:10-12 "Look! The days are coming,’ says Jehovah, ‘when I will make with the house of Israel and with the house of Judah a new covenant. It will not be like the covenant that I made with their forefathers on the day I took hold of their hand to lead them out of the land of Egypt, because they did not remain in my covenant, so I stopped caring for them,’ says Jehovah. ‘For this is the covenant that I will make with the house of Israel after those days,’ says Jehovah. ‘I will put my laws in their mind, and in their hearts I will write them. And I will become their God, and they will become my people. “‘And they will no longer teach each one his fellow citizen and each one his brother, saying: “Know Jehovah!” For they will all know me, from the least to the greatest of them. For I will be merciful toward their unrighteous deeds, and I will no longer call their sins to mind.’”

So then, as to the question you posted;

We want to keep in mind first that Jesus came and fulfilled the law.

Mth 5:17 "Do not think I came to destroy the Law or the Prophets. I came, not to destroy, but to fulfill."

So with that in mind we can look closer at Heb and Jer to determine what is meant by a new covenant. The scriptures read; "It will not be like the covenant that I made with their forefathers on the day I took hold of their hand to lead them out of the land of Egypt" So we understand that this covenant spoken of is "like" that one that was made with Israel when Jehovah brought them out of the land of Egypt.

First we can understand that this new covenant is not that one Jesus made with his disciples, for that one with his disciples was for rulership over his kingdom, and that one with Israel of old was for a promised land. Therefore this new covenant is more in line with the first, in that of a promised land flowing with milk and honey.

Second, this new covenant represents Jehovah's law. Being "like" the Law of Moses, but instead of written on stone tablets, it will be written on "their hearts".

Heb 10:10 "I will put my laws in their mind, and in their hearts I will write them."

So basically this "law" is that which is upon their hearts, and the fulfillment of Jehovah's promise to bring Israelite's into that promised kingdom. These Israelite's representing the Israelite's of the heart, the entire tree of Hebrew and Gentile.

The fulfillment of this covenant that is written on their hearts is as we read here;

Heb 8:10 "And I will become their God, and they will become my people."

Our Father fulfills his new covenant of bringing his people into a promised land, "like" that which was given to the Israelite's of old, but this time it will be fulfilled in their hearts, when they become his people, fulfilling his law, not on tablets, but on their hearts. Not a law to be obeyed by sinful man, which the Israelite's of old failed to do, but one that is fulfilled when these ones no longer disobey his law, at all...