When - After 85 days from now (http://www.bitcoinblockhalf.com/)
Why - The successful miner finding the new block is rewarded with newly created bitcoins and transaction fees.[57] As of 28 November 2012,[58] the reward amounted to 25 newly created bitcoins per block added to the block chain. To claim the reward, a special transaction called a coinbase is included with the processed payments.[10]:ch. 8 All bitcoins in existence have been created in such coinbase transactions. The bitcoin protocol specifies that the reward for adding a block will be halved every 210000 blocks (approximately every four years). Eventually, the reward will decrease to zero, and the limit of 21 million bitcoins[note 11] will be reached c. 2140; the record keeping will then be rewarded by transaction fees solely  ( Supply section of - https://en.wikipedia.org/wiki/Bitcoin )