Answer
The rule that five seconds elapse for each mile of distance is approximately correct.
Work Step by Step
We can use $340~m/s$ as the speed of sound.
Since the time it takes the light to reach us is negligible, we can ignore the travel time of the light. We can find the time it tkes a sound wave to travel a distance of $1.6~km$:
$t = \frac{d}{v} = \frac{1600~m}{340~m/s} = 4.7~s$
The rule that five seconds elapse for each mile of distance is approximately correct.
