How long did the Big Ear telescope's observation window last for the specific patch of sky where the 6EQUJ5 signal was recorded?

Approximately 72 seconds

The duration for which the Big Ear telescope could collect data from any specific celestial point was rigidly fixed, dictated by celestial mechanics—specifically, the rate at which the Earth's rotation carried that region out of the instrument's line of sight. This observation window lasted for about 72 seconds. This means the entire recorded event, represented by 6EQUJ5, occurred within this brief window. The fact that the signal stopped after 72 seconds is attributed to this mechanical limitation of the telescope's design relative to the Earth's movement, not necessarily the duration of the actual transmission event.