Your call credit remains valid for 12 months. The extra data and free minutes that you get with a top up are valid for 30 days.
So be sure to top up your card at least once every 12 months or you will lose your remaining call credit and your SIM card will be deactivated. Then your SIM card can no longer be used, and you need a new prepaid SIM card.
Log on to your customer zone to check the validity of your call credit.