You can find out when your current promotional rate on your credit card expires on the 'Your interest explained' section of your statement.
You'll find the information relating to your promotional rates, including the amount you have on a promotional rate and the date it will expire.
Once a promotional rate period has come to an end on your credit card, interest will be charged at the current standard interest rate that applies to that transaction.
Remember, to ensure you do not lose any promotional rates early you need to make at least the minimum payment, on time, every month.