Albert Barnes Commentary 2 Kings 17:5

"Then the king of Assyria came up throughout all the land, and went up to Samaria, and besieged it three years." — 2 Kings 17:5 (ASV)

All the land - This refers to Shalmaneser's second invasion (723 B.C., his fifth year), which is contrasted with the first. While the first invasion had afflicted only part of the country, this one extended to the whole land.

Three years - This period was from the fourth to the sixth year of Hezekiah’s reign and from the seventh to the ninth of Hoshea’s. This is two years by our reckoning, but three years according to the Hebrew method.

This was a long time for such a small place to resist the Assyrians, but several factors contributed to the delay. Samaria was favorably situated on a steep hill, and it is probable that Sabaco, the Egyptian king, made some attempts to relieve his vassal. The war with Tyre must have also distracted Shalmaneser. Furthermore, there is reason to believe that before the capture was complete, a revolt had broken out in Nineveh. This would have demanded Shalmaneser’s chief attention, though it did not cause him to abandon his enterprise.