SB 4.9.22

प्रस्थिते तु वनं पित्रा दत्त्वा गां धर्मसंश्रयः ।
षट्त्रिंशद्वर्षसाहस्रं रक्षिताव्याहतेन्द्रियः ॥२२॥

prasthite tu vanaṁ pitrā
dattvā gāṁ dharma-samśrayaḥ
ṣaṭ-trimśad-varṣa-sāhasraṁ
rakṣitāvyāhatendriyaḥ

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

Text

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.

Purport

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.

Srimad Bhagavatam. Canto 4: Creation of the Fourth Order

Chapter 9: Dhruva Maharaja Returns Home. Text 22

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.