Signal speed in fiber is about ⅔ of that in vacuum (https://en.wikipedia.org/wiki/Optical_fiber#Refractive_index), but fiber won’t be straight-line between sender and receiver, light doesn’t move in a straight line inside the fiber, and the switched adds delays.

https://www.pingdom.com/blog/theoretical-vs-real-world-speed...: “you should probably double the “ideal” response times shown above for a more realistic target to aim at“

So yes, ⅓ of light speed in vacuum seems a decent heuristic.

Speed of light in glass is about 2/3 of the speed of light in a vacuum (refractive index of glass is around 1.5).

Round trip latency matters here, which would get you down to 150km without any slowdown through fiber.