72 seconds is the duration of time that any particular patch of sky is in range for Big Ear to listen to. To offer another imperfect analogy, imagine yourself on a train in a pitch black tunnel. There's a break in the tunnel that lasts for 72 seconds at your current speed. The light signals you noticed for those 72 seconds didn't necessarily last for only 72 seconds, they were simply not visible after you went back into the tunnel.
