I recently bought a second hand car (Porsche Cayman S) and I was looking at the disk rotors.
Do you think that it's time to replace the front disk rotors of my car?
Unless the rotor shows obvious signs of damage there is no way to tell by a photo. The only way to be sure is to measure the rotor with a micrometer. The most accurate method is with a micrometer specifically designed to measure rotors. The contact areas of the micrometer are pointed, this allows measurement at the base of any grooves. After the smallest measurement is recorded it has to be compared with the manufacturers minimum thickness specification for your particular vehicle. It can then be determined if the rotor can be machined, often referred to as "turning the rotors". With the abundance of cheap imported rotors, in many cases replacement is a better option than resurfacing. My experience has been that the rotors turned to near their minimum warp quickly and end needing to be replaced.