Albert Barnes Commentary Psalms 61:6

"Thou wilt prolong the king`s life; His years shall be as many generations." — Psalms 61:6 (ASV)

Thou wilt prolong the king’s life — literally, “Days upon the days of the king You will add;” that is, You will add days to those which You have already permitted him to live. The language does not necessarily mean that he would have a long life, but that he would still be permitted to live.

He had feared death. He knew that his life was sought by those who were engaged with Absalom in the rebellion. At first, it was uncertain what the outcome would be. He had fled for his life. But now, in answer to prayer, he felt assured that his life would be preserved; that he would be permitted to return to his home and his throne; and that as king—as the sovereign of his people—he would be permitted to honor God.

And his years as many generations — In the margin, as in Hebrew, generation and generation. This probably means that he would be permitted to live longer than the ordinary time of a generation; that he would live as if one generation—or as if one ordinary lifetime—were added to another, so that he would live through successive generations of men. The average life of a generation is about thirty years. David is supposed to have lived from 1085 before the Christian era to 1016 BC, or 69 years, which would reach a third generation. This is a more natural interpretation of the passage than to suppose that he refers to an “ideal” king, or that his dynasty would continue for many generations.