Bitcoin is built so that this reward is halved every 210,000 blocks solved.  The idea is as bitcoin grows the transaction fee’s become the main part of the reward and the introduction of new bitcoin’s slows down to a trickle.  This also means that there will only ever be 21,000,000 bitcoins in circulation.

Well, in less than 4 days the block count will reach the first of these 210,000 block milestones and the reward for solving a Bitcoin block will half from 50BTC to 25BTC. (Have a look at bitcoinclock.com)

via Halving day is almost upon us! | Mineforeman.