Your Pay&Go card is valid 12 months as from when you make or receive your first call. That means that you have one year to use up your calling credit or top up your card.
During the last month in which your top-up card is valid, you will hear a notification message whenever you make a call.
After the expiry date you will no longer be able to make calls, text, etc. At that point you still have 7 days to top up so you can use your Pay&Go card again.