This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
... the blockchain, the more difficult it is to reverse. After a transaction is buried six or so blocks deep in the blockchain (about an hour), the probability of a successful double spend drops cl ose to zero.

But why all the fuss about about unconfirmed transactions? While most Bitcoin users can afford to wait an hour for their payments to confirm, there are some business mo
dels which cannot. Retail is the most obvious example. Customers need to be able to pay the cashier and leave the store instantly. They can’t be required to wait around for 10 minutes or longe...
read full book block explorer