Jews themselves do not teach nor understand Jeremiah's 70-year prophecy as referring to the time we remained in exile.
The English rendition of this text from the NJPS Tanakh reads:
For thus said the Lord: When Babylon’s seventy years are over, I will take note of you, and I will fulfill to you My promise of favor—to bring you back to this place.
In other words, the prophet says that God has set a time limit for Babylon of 70 years. When did these years actually begin and why did they begin at this point? The prophet never says. Did they end when Israel was released? Did they end when the famous "writing on the wall" of Daniel was seen? Was it when Babylon fell to its enemies? Or was it when the first exiled Jew set foot back in Israel? Maybe it was when the Temple was built? Nobody knows any of this, at least not Jews.
The 70 years determination is merely a declaration similar to Genesis 6:3 where God says people has been given a limit of 120 years. This is determined to mean the limit of a lifespan, but just a few verses later Noah lives over 900 years. Jacob lives to be 147 years old. Then is this verse in Genesis Saying there will be only 120 years until the Flood of Noah? If so, when did those years start to count? Are they lunar years? If so, do they follow the Jewish lunar calendar with its leap years or that of a more universally accepted "year" of all the earth that has since been forgotten?
These verses have no specifics regarding what the years being discussed involved. They do share the same feature of God being spoken of setting a limit, but nothing more.
I still have to wonder how and why Christians like the JWs make such big deals over making precise determinations over them. Especially regarding the Babylonian exile, since we Jews experienced it and even we don't have precise information on Jeremiah's meaning, what gives others greater insight or over-concern about this verse? So somehow God gives the prophecy to Jews but only Gentiles get to know it's actual meaning? Yeah, right.