The classic Bitcoin client will display the transaction as "unconfirmed" until 6 confirmations are received (6 blocks found). Sites or services that accept Bitcoin to pay for their goods or services may set their own limits on the number of blocks required to confirm a transaction. The number 6 was not chosen by chance: it is based on the theory that the likelihood that an attacker will be able to accumulate more than 10% of the network hashrate to fake transactions is small, and that a negligible risk (less than 0.1%) is acceptable.