I have OPEL ASTRA. I want to know when should I change the tires (After how much KM) and shall I change the spare tire also.

Regards

Rotate and balance tires every 10,500 km. Change tires when the tread depth approaches 2 mm, or 7 years, whichever comes first. Tires start dry rotting around 7 years and can become unsafe, especially in hot climates. Always ensure the spare tire is in great shape.