Most Proof of Work (POW) coins such as Bitcoin, Litecoin and many others have pre-defined block heights that they will cut the block reward in half. This means that the amount of new coins being minted daily will be half as much. The reduction in daily production typically is a small portion of daily volume and thus not really significant in the supply and demand arena.
It becomes challenging for miners (Who mint new coins and process transactions) to provide the same services for half the reward. The design expects that transaction activity eventually will provide enough income to pay miners high electricity costs once the block rewards are gone.
A lot of coins follow Bitcoins schedule which results in a halving approximately every 4 years but this absolutely depends on the criteria specified by the developer when they launched the coin.
You can usually find the block height for halving’s on the company/coin ANN page on bitcointalk or on their homepage (Search coin-name ANN). Current Block height can be found on any block explorer for that coin (Search coin-name block explorer). Block height is just the number of blocks it has processed since inception. A block is a block of transactions, a block of transactions is processed on a regular basis so the block height generally retains the same incline/growth so the halving’s are fairly predictable in timing.