The halving is a recurrent event, every 4 years the reward that the miner gets to resolve the Proof of Work is being cut in half.
More precisely: the reward is being cut in half every 210.000 blocks. A block is mined every 10 minutes. That’s why 210.000 blocks are equivalent to 1458 days, that are equivalent to 3.99 years!
At this very moment, the blockchain has 629.992 blocks. When a miner will find a new block with the height 630.000, 12 years will pass from the genesis block, and the reward will be 6.25 bitcoins per block instead of 12.5 bitcoins.
