SB 4.9.22

prasthite tu vanaṁ pitrā
 dattvā gāṁ dharma-saṁśrayaḥ
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.