Existence of material enjoyment can endure as long as life is fresh, but at the fag end of life everything becomes tasteless, and everything appears to be vain and painful
SB Canto 3
Consequently, although his duration of life gradually came to an end, his long life, consisting of a Manvantara era, was not spent in vain, since he ever engaged in hearing, contemplating, writing down and chanting the pastimes of the Lord.
As freshly prepared food is very tasteful but if kept for three or four hours becomes stale and tasteless, so the existence of material enjoyment can endure as long as life is fresh, but at the fag end of life everything becomes tasteless, and everything appears to be vain and painful. The life of Emperor Svāyambhuva Manu, however, was not tasteless; as he grew older, his life remained as fresh as in the beginning because of his continued Kṛṣṇa consciousness. The life of a man in Kṛṣṇa consciousness is always fresh. It is said that the sun rises in the morning and sets in the evening and its business is to reduce the duration of everyone's life. But the sunrise and sunset cannot diminish the life of one who engages in Kṛṣṇa consciousness. Svāyambhuva Manu's life did not become stale after some time, for he engaged himself always in chanting about and meditating upon Lord Viṣṇu. He was the greatest yogī because he never wasted his time. It is especially mentioned here, viṣṇoḥ kurvato bruvataḥ kathāḥ. When he talked, he talked only of Kṛṣṇa and Viṣṇu, the Personality of Godhead; when he heard something, it was about Kṛṣṇa; when he meditated, it was upon Kṛṣṇa and His activities.
It is stated that his life was very long, seventy-one yugas. One yuga is completed in 4,320,000 years, seventy-one of such yugas is the duration of the life of a Manu, and fourteen such Manus come and go in one day of Brahma. For the entire duration of his life - 4,320,000 x 71 years—Manu engaged in Kṛṣṇa consciousness by chanting, hearing, talking about and meditating upon Kṛṣṇa. Therefore, his life was not wasted, nor did it become stale.