Jeremiah 31:38-40 KJV -  Behold, the days come, saith the LORD, that the city shall be built to the LORD from the tower of Hananeel unto the gate of the corner.  And the measuring line shall yet go forth over against it upon the hill Gareb, and shall compass about to Goath.  And the whole valley of the dead bodies, and of the ashes, and all the fields unto the brook of Kidron, unto the corner of the horse gate toward the east, shall be holy unto the LORD; it shall not be plucked up, nor thrown down any more for ever.
Okay, on the surface this looks like a typical prophecy for the major prophets, something about "Behold, the days come..." Indeed, that's the whole point of prophecy, a telling of the future. But how far into the future? Look at verse 40, the very end of it. "... nor thrown down any more for ever."
So just how long is forever? Ah, I think that phrase "forever" holds the key to understanding the time period for that particular prophecy. But first, a disclaimer: In no way am I stating my opinion as fact. It is purely just that, my opinion. I could be 100% wrong. So that's the disclaimer.
The short version? The prophecy refers to eternity. Here is why:
2 Peter 3:10-13 KJV -  But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.  Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness,  Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat?  Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness.
All those places, the brook Kidron, the hill Gareb, they are going to burn. When 2 Peter 3:12 says the heavens shall be dissolved and the elements shall melt with fervent heat, kiss planet Earth goodbye. For that matter, say goodbye and good riddance to the entire universe.
Revelation 21:1-2 KJV -  And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.  And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband.So, in place of the old heaven and the old earth, we have a new heaven and a new earth. I suspect, and again, this is pure opinion, the new earth will bear a resemblance to the old one. Jerusalem will be there for eternity, so is is possible, perhaps, that so will those other landmarks mentioned in the Book of Jeremiah. Jerusalem will be exalted above all other cities, those other cities enjoying their existence on the new earth, while New Jerusalem floats above them all.
So, those valleys, the fields, all those landmarks there in the Book of Jeremiah, they will be reproduced in heavenly splendor and glory; free from the corruption of sin and disease, placed on the map of eternity never to be moved again. If it a landmark, and it says "forever", it is referring to the New Heaven and New Earth mentioned in Revelation 21. That's my take on it.