How many studs per second does this travel by?

Light is ill-defined in the context of ROBLOX, which simulates the physics of the real world rather than emulating reality, which would be computationally intractable.

You could define light in ROBLOX as the pixels rendered to the display or make an analogy between a ray of light (a single photon) and the casting of a Ray in ROBLOX.

Either way, the speed would depend on the underlying hardware used for the computation and specific properties of the simulation in question. e.g. transparent parts slow down pixel calculations, and the speed of ray intersections depends on the quantity and density of objects in the workspace.

Interestingly, the efficacy of your own CPU is limited (indirectly) by the speed of light in the real world - your computer is only as fast as physics allows it to be - and rendering pixels or calculating ray intersections depends on the abilities of your processor, so you could say that light in ROBLOX travels as fast as light in the real world allows it to.

Edit: For an empirical observation, you could time how long it takes for a ray intersection to be calculated given different quantities and arrangements of parts - perhaps build a graph of how the speed of a ray changes with increased part count, thus approximating the 'speed of light' in ROBLOX.

It would be interesting, actually, to see if ray intersections with objects close to the ray origin are faster to compute than far away intersections. (Assuming the length of the ray and number of parts stays constant). You could then measure the speed of a ray with respect to distance, rather than just how long it takes to calculate an arbitrary intersection.