Albert Barnes Commentary 2 Kings 19:29

"And this shall be the sign unto thee: Ye shall eat this year that which groweth of itself, and in the second year that which springeth of the same; and in the third year sow ye, and reap, and plant vineyards, and eat the fruit thereof." — 2 Kings 19:29 (ASV)

The prophet now addresses Hezekiah once more and gives him a sign, or token, by which he and his people could be assured that Sennacherib was indeed bridled and would no longer trouble Judah. This was a sign of the land's continued freedom from attack for the entire remainder of Sennacherib’s reign—a period of seventeen years.