At least 20 years
30 And Hoshea the son of Elah made a conspiracy against Pekah the son of Remaliah, and struck him and put him to death and became king in his place, in the twentieth year of Jotham the son of Uzziah.
2 Kings 15:30  NASBu

16 years
33 He [Jotham] was twenty-five years old when he became king, and he reigned sixteen years in Jerusalem; and his mother’s name was Jerusha the daughter of Zadok.
2 Kings 15:32  NASBu 

SAB Contradiction 260

The Problem
These texts seem to reveal a clear Bible Contradiction. King Jotham reigned sixteen years and in another text is spoken about the twentieth year of his government. Seems a little bit confusing. Doesn’t it?

Concealed information
The critic only mentioned two texts. But the Bible gives much more information about king Jotham. And by the method of the critic to only give two texts he is in fact concealing material relevant to understanding the texts. What is it that remains hidden?
Jotham was the son of king Uzziah who in the last period of his life suffered leprosy and in that time Jotham took over the government over Judah. He got the title of king only after the death of his father Uzziah.

The period of Jotham’s stand-in
And so it is absolutely correct when the Bible speaks about the twentieth year of Jotham, while he was only king for 16 years. The Bible doesn’t say how long Jotham was a stand-in for his father, but about 8 years is a reasonable estimate.

Meaning of words
In verse 30 the absence of the phrase “l’malkut” (of the kingship) is important: “… in the twentieth year of Jotham” the expression “kingship” is not added. In most reports about kings it is not necessary to add, as it is clear from the context. Sometimes it is added, e.g. 2 Chronicles 35:19, “In the eighteenth year of Josiah’s reign [kingship] this Passover was celebrated.” Anyway, by not mentioning the phrase “l’malkut”, the Bible remains in perfect harmony.

No Bible Contradiction