According to Hindu scripture, Kali Yuga (the Age of Kali) began at the end of Krishna's bodily lifespan (approximately 5100 years ago, 3100 BC) and will last exactly 432,000 years - placing its conclusion near the middle of the 431st millennium AD. Kalki, the 10th and final avatar of Vishnu, is expected to appear at this time, riding a white horse and wielding a flaming sword with which to strike down the wicked.
Kali Yuga is the last of 4 Yugas; upon its conclusion, the world will "reboot" into a new Satya Yuga (Golden Age.) This involves the end of the world as we know it and the return of Earth to a state of paradise.