How long does a spike take to cross the brain or reach a foot? · Grey Matter

It depends on the fibre.

How long does a spike take to cross the brain or reach a foot?

It depends on the fibre. The fastest myelinated motor and muscle-sense fibres conduct at about 80 to 120 m/s, so a command covering a metre to the foot takes around 10 ms; thin unmyelinated pain fibres at about 1 m/s need a second for the same distance, which is why a burn is felt after the first sharp prick. Inside the brain most axons are short and slower, and a signal between hemispheres through the corpus callosum takes from a few to a few tens of milliseconds, depending on the fibre. Synaptic delays at each relay add to the conduction time.