It has no expiry date. Note, however, you can lose your right to permanent residence if you live outside Spain for more than 2 consecutive years. Holders of the Residency Card for Family Members of EU Citizens lose their right to residence if they leave Spain for a period of more than six months within a year unless the absence is for certain and justifiable reasons such as military service, employment or illness.
Unlike the registration certificate that is required in many countries, the permanent residence document is not mandatory.
However, it is useful for other procedures and saves you having to prove again that you have work, sufficient funds, medical insurance, etc.