When will he come? He will come when everybody awakens unconditional love inside the heart. There is no date for this coming. God is not bound by any time. He is the Lord of time, himself.
The white horse symbolizes the purity together with a controlled mind. In the Bhagavad Gita Krishna drives a chariot with five horses. These are the five senses that we have to control. The Kalki Avatar rides upon the mind itself. Not only the five senses are controlled but the mind itself is focused, self-pointed and governed by the Self. When the mind is controlled, pure love will awaken and the Lord will reveal Himself in the Heart of mankind.