light

Light travels 300,000 km in a second.

That is 300,000,000 m and thus 300,000,000,000 mm.

A gigahertz is 1/1,000,000,000 of a second.

        1            300,000,000,000     300
   -------------  *  ---------------  =  ---  =  300
   1,000,000,000            1             1

This means light travels 300 mm at one tick of the clock running at 1 gigahertz.

This is just shy of one foot.