There are two types of customers. It would depend on whether you were using the card for payment and paying off your balance every month, in which case we're loaning money for 51 days, or whether you're a borrower. If you're a borrower, just as with any other credit product, you're going to pay interest on the entire amount as of the date that you borrow it. So if you made the purchase on the 15th of the month, you'd pay interest on that full amount until it was paid off.
On May 28th, 2009. See this statement in context.