Thou wilt prolong the king’s life - literally, “Days upon the days of the king thou wilt add;” that is, Thou wilt add days to those which thou hast 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 apprehended 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 issue 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 - 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 b.c., 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.