If you had a continuous optical fiber cable with a length of 300,000 kilometers (km) and could see both ends simultaneously, you would indeed observe a light delay when a light source is switched on at one end and observed at the other end.
The speed of light in a vacuum is approximately 299,792 kilometers per second (km/s). However, when light travels through an optical fiber, its speed is slightly slower due to the refractive index of the fiber material. Typically, the speed of light in an optical fiber is around 200,000 to 230,000 kilometers per second (km/s).
Given the length of the optical fiber cable you mentioned (300,000 km), the light would take approximately 1.3 to 1.5 seconds to travel from one end to the other. This means that when the light source is switched on at one end, it would take around 1.3 to 1.5 seconds for the light to reach the other end, where it would be visible to an observer.
It's worth noting that this estimate assumes an ideal scenario with a straight, uninterrupted fiber cable and neglects any additional delays caused by signal propagation and processing within the fiber. In practical situations, there might be some additional delays or losses in the optical fiber, but the basic principle of light delay would still apply.