Jamieson, Fausset and Brown on Isaiah 39:6

6. days come--one hundred twenty years afterwards. This is the first intimation that the Jews would be carried to Babylon--the first designation of their place of punishment. The general prophecy of Moses (Le 26:33; De 28:64); the more particular one of Ahijah in Jeroboam's time (1Ki 14:15), "beyond the river"; and of Am 5:27, "captivity beyond Damascus"; are now concentrated in this specific one as to "Babylon" (Mi 4:10). It was an exact retribution in kind, that as Babylon had been the instrument of Hezekiah and Judah's sin, so also it should be the instrument of their punishment.