SB 4.9.22

prasthite tu vanaṁ pitrā
dattvā gāṁ dharma-saṁśrayaḥ
ṣaṭ-triṁśad-varṣa-sāhasraṁ
rakṣitāvyāhatendriyaḥ

prasthite—after departure; tu—but; vanam—to the forest; pitrā—by your father; dattvā—awarding; gām—the whole world; dharma-saṁśrayaḥ—under the protection of piety; ṣaṭ-triṁśat—thirty-six; varṣa—years; sāhasram—one thousand; rakṣitā—you will rule; avyāhata—without decay; indriyaḥ—the power of the senses.

After your father goes to the forest and awards you the rule of his kingdom, you will rule continuously the entire world for thirty-six thousand years, and all your senses will continue to be as strong as they are now. You will never become old.

In the Satya-yuga people generally lived for one hundred thousand years. Dhruva Mahārāja's ruling the world for thirty-six thousand years was quite possible in those days.