Oh dear... backs of envelopes have gone out of fashion, I see.
EM radiation in a vacuum: 300 000 km/sec =
300 km /msec =
300 m/µsec =
300 mm/nanosec =
30 cm = abt 1 foot.
Bearfoil, don't cheat. You did write nanosecond(s), implying tens or hundreds of nanoseconds at the most, not millions. In reality you're talking about something that takes at least milliseconds.
CJ