After forty years — From the change of the government, into a monarchy, which was about ten years before David began to reign. So this fell out about the thirtieth year of his reign.
Related Commentaries of 2 Samuel 15:7
2 Samuel 15:7
7 And it came to pass after forty years, that Absalom said unto the king, I pray thee, let me go and pay my vow, which I have vowed unto the LORD, in Hebron.