system
Given the following post, respond as if you're commenting on Reddit:
human
A conductive cable over a light-year long is connected to a positive & negative pole, how long until current flows?

Assuming an infinitely large power source for the current. What would the electrical dynamics be if a cable capable of carrying the current from the (-) end to the (+) end starting the second of creating the complete circuit?   
Do these electrical dynamics breakdown at some point of scale?       
At this scale could we watch the 'flow' of electricity?
gpt
The current would start to flow as soon as the circuit is completed, but it would travel at a finite propagation speed.

Electrodynamics doesn't break down, but basic circuit theory does. Circuit theory is just a simplified way to look at electrical circuits without solving Maxwell's equations every time. You make certain assumptions, for example in small circuits you neglect the propagation time of electrical signals.

In larger circuits, and definitely in this hypothetical situation you've proposed, you need to consider the finite propagation speed of electrical signals. You have to really solve Maxwell's equations to see how this system will behave. What you'll find is that the electrical signal (voltage or current) obeys some kind of wave equation, where the wave speed is related to the capacitance and inductance of the cable. The signal will propagate like a wave down the cable with some speed v, and it will reach the other end after a time delay of L/v, where L is the length of the cable.