Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 4
दशवर्षसहस्रान्ते पुरुषस्तु सनातन: ।तेषामाविरभूत्कृच्छ्रं शान्तेन शमयन् रुचा ॥ ४ ॥
daśa-varṣa-sahasrāntepuruṣas tu sanātanaḥteṣām āvirabhūt kṛcchraṁśāntena śamayan rucā
At the end of ten thousand years of severe austerities performed by the Pracetās, the Supreme Personality of Godhead, to reward their austerities, appeared before them in His very pleasing form. This appealed to the Pracetās and satisfied the labor of their austerities.
Performing ten thousand years of severe austerities does not seem a happy endeavor. Yet the devotees, the serious students of spiritual life, undergo such austerities to attain the favor of the Supreme Personality of Godhead. At that time, when the duration of life was very long, people could undergo severe austerities for thousands of years. It is said that Vālmīki, the author of Rāmāyaṇa, underwent meditational austerities for sixty thousand years. The Supreme Personality of Godhead appreciated the austerities undergone by the Pracetās, and He finally appeared before them in a pleasing form. Thus they all became satisfied and forgot the austerities they underwent. In the material world, if one is successful after hard labor, he is very pleased. Similarly, the devotee forgets all his labors and austerities as soon as he contacts the Supreme Personality of Godhead. Although Dhruva Mahārāja was only a five-year-old boy, he underwent severe austerities by eating simply dry foliage, drinking only water and taking no food. In this way, after six months he was able to see the Supreme Personality of Godhead face to face. When he saw the Lord, he forgot all his austerities and said, svāmin kṛtārtho ’smi: “My dear Lord, I am very pleased.” Of course, these austerities were performed in the Satya-yuga, Dvāpara-yuga and Tretā-yuga, but not in this Age of Kali. In this Kali-yuga, one can attain the same results simply by chanting the Hare Kṛṣṇa mahā-mantra. Because the people of this age are fallen, the Lord is kind enough to give them the easiest method. Simply by chanting the Hare Kṛṣṇa mantra, one can attain the same results. However, as Lord Caitanya Mahāprabhu points out, we are so unfortunate that we are not even attracted to chanting the mahā-mantra — Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.