Question: Was Jeremiah wrong when he said the Babylonian Exile would last 70 years?
Response: No. Jeremiah never said that the Babylonian Exile would last 70 years. Instead, he prophesized that Babylon would rule over the country of Judah for a period of 70 years, which it did. This is sometimes referred to as the Babylonian Captivity.
People often make the mistake of confusing the Babylonian Captivity with the Babylonian Exile. They were separate, though related, events. These two separate events began at different times, but ended at the same time.
For more information, visit the AboutBibleProphecy.com article at Seventy years of Babylonian rule.