When the reward for successfully mining a block of bitcoin is reduced by half, which occurs every four years. For instance, the initial reward for mining a Bitcoin block was 50 bitcoins, which was reduced to 25 in 2012 after the first halving and half again to 12.25 bitcoins after the next halving. This mechanism ensures a finite amount of coins are created for a crypto currency. The actual time span is not 4 years, but rather the amount of time taken to mine 210,000 blocks.