How do you so consistently think you'll get away with pronouncing such pompous ignorance?
Jeremiah predicted that Nebuchadnezzar would conquer Judea. He foretold how long the exile would be, and (as 2nd Chronicles 36:21 attests) he even knew the reason for the duration. What you clearly do not know is how the Sabbath for the Land figured into the system of Israel's national defense, but then you are in good company there.
That one was immediately following your response to me.
Now, "how long" ~ as long as disputations over the character of prophetic utterances is tolerated ~ that's how long.