Jack Reynolds
First time out of the vault

Now, I'm sure this has been done to death, but I want to add something to it, and Search isn't bringing up any previous iterations on this point.
So here we go.
Ferality in ghouls is not a degree of ghoulification. It's a type. For convenience, I'm going to refer to non-feral ghoulification as Type 1, and Feral ghoulification as Type 2.
Let's start at the beginning -- and the beginning of ghouls was not the bombs dropping. We know this because of characters like Desmond Lockheart, who underwent ghoulification before the bombs fell. So what was the beginning?
The New Plague.
The New Plague, for those who aren't familiar, was a global pandemic of a retrovirus. Possibly (in the Fallout universe, almost certainly) a bioweapon which either escaped containment or which was deployed deliberately -- if the latter, the purpose seems clear: To ease resource depletion via deliberate depopulation.
In response to the New Plague, the U.S. Government used samples of it to begin engineering the Pan-Immunity Virion; a version of the New Plague recoded to attack the New Plague -- and everything else. The Pan-Immunity Virion would have been administered to U.S. military forces, as well as personnel associated with U.S. government facilities.
Now, the Pan-Immunity Virion had side effects. Its mutagenic properties manifested unexpected mutations. Seeing this, the U.S. government started... playing with it. Eventually, this resulted in the development of the Forced Evolutionary Virus.
However: Not all people who survived the New Plague did so via the Pan-Immunity Virion. Because the original virus was, itself, mutagenic, some who survived it did so because their own immune systems were mutated by the New Plague and took on mutagenic function.
So, Type 1: Naturally survived the New Plague via mutagenesis as a natural adaptation.
Type 2: Survived the New Plague due to the mutagenic effect of the Pan-Immunity Virion.
Then the bombs drop.
Type 1 New Plague survivors' immune systems adapt to fight off radiation poisoning. It... takes a few days, maybe a couple weeks. The radiation destroys the outer dermal layers, exterior cartilaginous tissues like nose and/or ears. But the subject survives, with minimal loss of external tissues and with all brain functions totally unaffected. Non-feral ghouls.
Type 2 New Plague survivors also adapt to radiation -- but... the adaptation is less rapid and less robust, because their immune systems aren't naturally mutagenic like those of Type 1 survivors. In Type 2s, the radiation damage is therefore more serious and more insidious. The brain is damaged; maybe not immediately, maybe it imparts a more devastating equivalent of Alzheimer's disease. Feral ghouls.
Now, here's the tricky part: In Type 2s, feralization can be rapid, or it can take decades or centuries of cumulative exposure to tip a Type 2 over the edge.
Evidence against Feralization being exclusively a matter of amount or rapidity of exposure:
1. Jason Bright. Jason Bright is so massively irradiated that he has become a Glowing One and actively emits radiation. And yet... he's not feral.
2. NCRA Private Kyle Edwards: Edwards was irradiated so heavily and so rapidly that he, along with the rest of his squad in Camp Searchlight, pretty nearly "insta-"ghoulified. Also not feral; moreover, we can contrast him against the rest of his squad, who did go feral with the same dose and rapidity of radiation exposure.
The Conclusion:
Feral vs Non-feral isn't a matter of time but of type.
Ancillary Notes:
With ferality being a matter of cumulative exposure, characters like Cooper Howard are perfectly plausible as future ferals. Howard spent a great deal of his 219 years isolated from radiation sources. He's also a military veteran of Operation: Anchorage, which means that he very plausibly received the Pan-Immunity Virion, though this does not preclude that he had already defeated the New Plague naturally.
Furthermore: Based on this theory, because we saw that Rose McLean went feral, it's a certainty that, with sufficient radiation exposure, Lucy McLean will go feral as well.
So here we go.
Ferality in ghouls is not a degree of ghoulification. It's a type. For convenience, I'm going to refer to non-feral ghoulification as Type 1, and Feral ghoulification as Type 2.
Let's start at the beginning -- and the beginning of ghouls was not the bombs dropping. We know this because of characters like Desmond Lockheart, who underwent ghoulification before the bombs fell. So what was the beginning?
The New Plague.
The New Plague, for those who aren't familiar, was a global pandemic of a retrovirus. Possibly (in the Fallout universe, almost certainly) a bioweapon which either escaped containment or which was deployed deliberately -- if the latter, the purpose seems clear: To ease resource depletion via deliberate depopulation.
In response to the New Plague, the U.S. Government used samples of it to begin engineering the Pan-Immunity Virion; a version of the New Plague recoded to attack the New Plague -- and everything else. The Pan-Immunity Virion would have been administered to U.S. military forces, as well as personnel associated with U.S. government facilities.
Now, the Pan-Immunity Virion had side effects. Its mutagenic properties manifested unexpected mutations. Seeing this, the U.S. government started... playing with it. Eventually, this resulted in the development of the Forced Evolutionary Virus.
However: Not all people who survived the New Plague did so via the Pan-Immunity Virion. Because the original virus was, itself, mutagenic, some who survived it did so because their own immune systems were mutated by the New Plague and took on mutagenic function.
So, Type 1: Naturally survived the New Plague via mutagenesis as a natural adaptation.
Type 2: Survived the New Plague due to the mutagenic effect of the Pan-Immunity Virion.
Then the bombs drop.
Type 1 New Plague survivors' immune systems adapt to fight off radiation poisoning. It... takes a few days, maybe a couple weeks. The radiation destroys the outer dermal layers, exterior cartilaginous tissues like nose and/or ears. But the subject survives, with minimal loss of external tissues and with all brain functions totally unaffected. Non-feral ghouls.
Type 2 New Plague survivors also adapt to radiation -- but... the adaptation is less rapid and less robust, because their immune systems aren't naturally mutagenic like those of Type 1 survivors. In Type 2s, the radiation damage is therefore more serious and more insidious. The brain is damaged; maybe not immediately, maybe it imparts a more devastating equivalent of Alzheimer's disease. Feral ghouls.
Now, here's the tricky part: In Type 2s, feralization can be rapid, or it can take decades or centuries of cumulative exposure to tip a Type 2 over the edge.
Evidence against Feralization being exclusively a matter of amount or rapidity of exposure:
1. Jason Bright. Jason Bright is so massively irradiated that he has become a Glowing One and actively emits radiation. And yet... he's not feral.
2. NCRA Private Kyle Edwards: Edwards was irradiated so heavily and so rapidly that he, along with the rest of his squad in Camp Searchlight, pretty nearly "insta-"ghoulified. Also not feral; moreover, we can contrast him against the rest of his squad, who did go feral with the same dose and rapidity of radiation exposure.
The Conclusion:
Feral vs Non-feral isn't a matter of time but of type.
Ancillary Notes:
With ferality being a matter of cumulative exposure, characters like Cooper Howard are perfectly plausible as future ferals. Howard spent a great deal of his 219 years isolated from radiation sources. He's also a military veteran of Operation: Anchorage, which means that he very plausibly received the Pan-Immunity Virion, though this does not preclude that he had already defeated the New Plague naturally.
Furthermore: Based on this theory, because we saw that Rose McLean went feral, it's a certainty that, with sufficient radiation exposure, Lucy McLean will go feral as well.