Just to clarify - it's up to 56 days interest free credit. It really depends on when the transaction is posted relative to your billing date.
If your billing date is the 15th of the month, if you make a transaction on the 14th of August it will appear on your 15th of August bill and you'll probably have to pay it back around the start of September. But if you were to make a transaction on the 16th of August, it would appear on your 15th of September bill, and you wouldn't have to pay it back until the start of October.
People sometimes assume that it's 56 days exactly for all transactions - it isn't.
Discuss how they went here