This Bible contradiction is from the Skeptic's Annotated Bible.
At least 20 years.
2 Kings 15:30 View context
And Hoshea the son of Elah made a conspiracy against Pekah the son of Remaliah, and smote him, and slew him, and reigned in his stead, in the twentieth year of Jotham the son of Uzziah.
2 Kings15:32-33 View context
In the second year of Pekah the son of Remaliah king of Israel began Jotham the son of Uzziah king of Judah to reign.
Five and twenty years old was he when he began to reign, and he reigned sixteen years in Jerusalem. And his mother's name [was] Jerusha, the daughter of Zadok.