"The reward for solving a block is automatically adjusted so that roughly every four years of operation of the Bitcoin network, half the amount of bitcoins created in the prior 4 years are created. 10,500,000 bitcoins were created in the first 4 (approx.) years from January 2009 to November 2012. Every four years thereafter this amount halves, so it will be 5,250,000 over years 4-8, 2,625,000 over years 8-12, and so on. Thus the total number of bitcoins in existence will never exceed 21,000,000. Blocks are mined every 10 minutes, on average, and for the first four years (210,000 blocks) each block included 50 new bitcoins. As the amount of processing power directed at mining changes, the difficulty of creating new bitcoins changes. This difficulty factor is calculated every 2016 blocks and is based upon the time taken to generate the previous 2016 blocks"