Albert Barnes Commentary 2 Kings 10:30

"And Jehovah said unto Jehu, Because thou hast done well in executing that which is right in mine eyes, [and] hast done unto the house of Ahab according to all that was in my heart, thy sons of the fourth generation shall sit on the throne of Israel." — 2 Kings 10:30 (ASV)

And the Lord said to Jehu - This was probably spoken by the mouth of Elisha. To a certain extent, Jehu’s actions were acts of obedience, for which God saw fit to assign him a temporal reward.

Your children... - This was accomplished in the persons of Jehoahaz, Joash, Jeroboam, and Zechariah—the son, grandson, great-grandson, and great-great-grandson of Jehu (compare the marginal references). No other family sat on the throne of Israel for so long. The house of Omri, which furnished four kings, held the crown for only three generations and for less than 50 years, while the house of Jehu reigned for five generations and for more than 100 years.