How big will the block chain become? How big can the block chain become? It's already 7 GB after a couple years!
The block chain length in number of blocks will grow indefinitely with one block being created for 10 minutes on average.

The actual size of the whole chain will strongly depend on the following two aspects:


the number of transactions


The number of transactions in a block determine the size of the block. So, the more transactions are made in the network, the more information there has to be stored.


how the block chain will be stored across clients


Since the speed with which the block chain grows has been slightly increasing since the beginning of Bitcoin and the block chain is already significantly big, some of the Bitcoin developers have been considering to implement a solution for this that would make it unnecessary to keep a history since the beginning of time.

Currently already the standard Bitcoin client compresses the block chain for storage. The older blocks become, the more they are compressed. This means that you required less disk space to store the block chain, but also that it takes longer to start building it from scratch. Other clients have to decompress it to share it with you and you will have to compress everything you receive. This results in the fact that new clients often need multiple days to get in sync with the network.