Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 22

Sanskrit:

प्रस्थिते तु वनं पित्रा दत्त्वा गां धर्मसंश्रय: ।षट्-त्रिंशद्वर्षसाहस्रं रक्षिताव्याहतेन्द्रिय: ॥ २२ ॥

ITRANS:

prasthite tu vanaṁ pitrādattvā gāṁ dharma-saṁśrayaḥṣaṭ-triṁśad-varṣa-sāhasraṁrakṣitāvyāhatendriyaḥ

Translation:

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.