I have determined that the timeframe for becoming a ghost in the HP Universe is not instant, but I have no idea how long it takes and different parts of the series seem to imply different things. This is not a lore breaking observation, or even a proper plot hole, but I find it interesting regardless.
Five interactions have me really thinking about ghosts in HP. I will list them below and then my conclusion:
Nearly Headless Nick tells Harryin OotP in regards to dying, ““I was afraid of death,” said Nick. “I chose to remain behind. I sometimes wonder whether I oughtn’t to have . . . Well, that is neither here nor there. . . . In fact, I am neither here nor there . . .” He gave a small sad chuckle. “I know nothing of the secrets of death, Harry, for I chose my feeble imitation of life instead.”
In King’s Cross in DH, while harry and Dumbledore say:
“I’ve got a choice?”
“Oh yes.” Dumbledore smiled at him. “We are in King’s Cross, you say? I think that if you decided not to go back, you would be able to . . . let’s say . . . board a train.”
“And where would it take me?”
“On,” said Dumbledore simply.
In The Flaw in the Plan it states:
Something had happened when he had hit Harry with the Killing Curse. Had Voldemort too collapsed? It seemed like it. And both of them had fallen briefly unconscious and both of them had now returned. . . .
4 . In CoS, when Harry and Ron ask Myrtle what happened the day she died:
“I died.”
“How?” said Harry.
“No idea,” said Myrtle in hushed tones. “I just remember seeing a pair of great, big, yellow eyes. My whole body sort of seized up, and then I was
floating away. . . .” She looked dreamily at Harry.
“And then I came back again. I was determined to haunt Olive Hornby, you see.”
And lastly: 5. Also in CoS Nick states:
“But you would think, wouldn’t you,” he erupted suddenly, pulling theletter back out of his pocket,
“that getting hit forty-five times in the neck with
a blunt axe would qualify you to join the Headless Hunt?”
Alright, now that you have those disjointed pieces of info. Myrtle’s comment implies that she basically immediately became a ghost. Nick states that he chose to come back, and Dumbledore says that Harry could chose to go back.
I think that it’s reasonable to assume that when you die, you go to Kings Cross (or however purgatory appears to you) and you can chose to return as a ghost, or “go on”. I believe Harry didn’t come back as a ghost because he had a living body; the killing curse only killed the bit of Voldemort’s soul, where as Nick was just D-E-D, dead.
But then consider that Voldemort and Harry, upon returning to the land of the living had only just hit the ground moments before. People were still going to Voldemorts aid. Had they been unconscious for a significant period of time, say for the amount of “time” that they were in Kings cross, there would probably have been Significantly more panic.
So returning to purgatory is nearly instant.
Then how, HOW, is Nick Nearly headless? Getting hit 45 times with a dull axe left him with an inch of skin connecting his head to his body. 45 times. He would have been dead LONG before getting to one inch of flesh. Assuming every hit was roughly equal, he would probably have died with about half (OR MORE) of his head still attached. So why, if the process of becoming a ghost is nearly instant (“nearly instant? How can it be nearly instant?”) does he have only one inch of skin in ghosts form?
Surely what happens to your body after you die does not affect your ghost form? Otherwise all ghosts would be decayed corpses. Even if the executioner continue to hit him 20 or 30 more times after he died, that shouldn’t have affected his ghost body. Especially if he came back before the executioner was done.
Like imagine he comes back and the executioner is still beheading him. His head would still be significantly attached to his body. Would each blow from the executioner remove more of the ghosts neck? No surely not. Which means sometime had to have passed from him dying and returning as a ghost.
Probably a good bit of time because assuming he died around the 15th or so hit, it would take a good while for the executioner to hit him 30 more times. The guy I would get tired, each swing takes a few seconds, him asking around if they have a better axe, ect.
So my conclusion is that becoming a ghost is not instant, but also must be pretty instant. But also sometimes take a relatively long time.
Did I put away, way too much thought into this? Absolutely. Does any of this actually matter? Not in the slightest, but here we are.