How Does Transaction Confirmation Time Impact the Risk of a Double-Spend?

The risk of a successful double-spend decreases dramatically as the number of block confirmations increases. A transaction with zero confirmations is highly vulnerable, especially to a "race attack." Once a transaction is included in a block and that block is followed by subsequent blocks (confirmations), the cost for an attacker to rewrite the history and execute a double-spend grows exponentially due to the required Proof-of-Work.

Six confirmations is the widely accepted standard for high-value transactions.