The speed of light is 299,792,458 meters per second (m/s). To convert this to millimeters per second (mm/s), we need to multiply by 1000 (since there are 1000 millimeters in a meter):

299,792,458 m/s * 1000 mm/m = 299,792,458,000 mm/s

Therefore, the speed of light is 299,792,458,000 millimeters per second.