It is difficult to reach any concrete conclusions about how long it takes Farquhar to die in Ambrose Bierce's short story "An Occurrence at Owl Creek Bridge" because, though the story is rich with details, it actually contains very contradictory details. The length of time it takes to die by hanging depends on the method used. The ideal method of hanging is via a long drop, which breaks the victim's neck and only takes between 15 and 20 minutes to cause complete death. The non-ideal, excruciating method is via a short drop, which causes death by strangulation and does not break the victim's neck. The contradictions found in Bierce's short story concern the fact that Peyton Farquhar seems to be being strangled plus seems to have had his neck broken, and both are not possible scenarios.
Descriptions of the apparatus that the Union soldiers rigged, lead a reader to deduce that Farquhar is being executed via a long drop. The soldiers are standing on the bridge Farquhar had tried to burn down to prevent the Union soldiers from advancing. Two private soldiers have removed and loosened planks of the bridge so that only the captain's weight holds down the plank upon which both the captain and Farquhar are standing. Upon the signal, the captain steps off the plank, and the plank drops beneath Farquhar's feet, sending him plummeting towards the river below. Since bridges generally stand fairly high above the water they are bridging, especially if the water is prone to flooding, we know Farquhar's drop from the bridge is fairly long. The final line of the story is also consistent with being hanged via a long drop, since only a long drop will break the victim's neck:
Peyton Farquhar was dead; his body, with a broken neck, swung gently from side to side beneath the timbers of the Owl Creek bridge.
If his neck was broken during the hanging, then he would have immediately lost consciousness because blood pressure drops. After loss of consciousness, brain death occurs within a few minutes; it will then take 15 to 20 minutes for total death to occur.
Also consistent with a long drop is Bierce's description that as he "fell straight downward through the bridge he lost consciousness and was as one already dead." Yet, the inconsistent descriptions happen immediately after this sentence. According to the author, Farquhar then regained consciousness and felt the excruciating pain related to strangulation. The reader can also deduce that his fantasies of escaping and returning home to his wife occurred while his brain was still functioning during strangulation. If he did indeed die via strangulation, he would have lost consciousness in 7 seconds to a full minute; then, total death would have again occurred in 15 to 20 minutes.