prasthite tu vanam pitra
dattva gam dharma-samsrayah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In the Satya-yuga people generally lived for one hundred thousand years. Dhruva Maharaja’s ruling the world for thirty-six thousand years was quite possible in those days.