That is very roughly... it is about 2/3 c and can be calculated by the refractive index of quartz glass vs air (e.g. SiO2 has refractive index between 1.55 bad

That is very roughly... it is about 2/3 c and can be calculated by the refractive index of quartz glass vs air (e.g. SiO2 has refractive index between 1.55 bad and 1.4 good for the speed) e.g. 1/1.55 ~ 0.645 and 1/1.4 ~ 0.714. According to "Main Parameters" section of [0] it seems to be 1/1.44 for the silica used in actual fibers. A good approximation would be 200 km/ 1 ms instead of 300 km/ 1 ms for speed of light in a vacuum/ microwave links. [0] https://www.rp-photonics.com/fibers.html https://www.rp-photonics.com/fibers.html