How long was he king? Aside from being taken captive by Yehoash (king over Israel kingdom) he also stayed 15 years in Lachish. My question is how long did he actually rule - till he was taken captive.
Seder Olam states that Amaziah reigned for 29 years until he died in Lachish. Rashi states that the last 15 years of his life, he spent in Lachish though he was still technically king and this was included in the 29 years.
He was twenty-five years old when he became king, and he reigned in Jerusalem twenty-nine years. His mother's name was Jehoadan of Jerusalem.
2915 -845 2 Amaziah son of Joash reigns II Kings 14:1 Y2 Jehoahaz
2917 -843 17 Jehoahaz son of Jehu dies II Kings 13:1 reigned 17Y
2917 -843 40 Joash is murdered by 2 servants II Kings 12:22 reigned 40Y
2919 -841 Black Obelisk of Shalmanezer III Historical
2929 -831 16 Jehoash son of Jehoahaz dies II Kings 13:10 reigned 16Y
2929 -831 14 Jeroboam son of Jehoash reigns II Kings 14:23 Y15 Amaziah
2940 -820 -16 Azariah son of Amaziah born II Kings 14:21,15:2 reigned 16Y old
2944 -816 29 Amaziah flees to Lachish; killed there II Kings 14:2,17 reigned 29Y
2956 -804 27 Azariah son of Amaziah reigns II Kings 15:1-2 Y27 Jeroboam
Thus we see that Amaziah reigned from 2915 to 2944 which is 29 years. However, Rashi says that this 29 years includes the 15 years that he spent in Lachish.
25 And Amaziah the son of Joash the king of Judah lived fifteen years after the death of Joash the son of Jehoahaz the king of Israel.
27 And from the time that Amaziah turned away from following the Lord, they conspired against him in Jerusalem, and he fled to Lachish. And they sent after him to Lachish and assassinated him there.
and he fled to Lachish: a fortified city against which Sennacherib waged war. He fled to Lachish, where he remained fifteen years until they followed him to Lachish and assassinated him there. And during those fifteen years that he was in Lachish, Jecoliah, Uzziah’s mother, reigned in his stead. This is how it appears to me. In the commentary of Rabbi Joseph (Kara), I saw that during those fifteen years that he was in Lachish, Uzziah his son reigned in his stead, but it is incorrect to interpret it in this manner, for it is written immediately following (26:1): “who was sixteen years old.” We find that when his father fled, he was only one year old. Moreover, if this is the case, it should say, “took Uzziah and made him king instead of his father during his lifetime.” Rather, Scripture intimated that he did not reign during his father’s lifetime.