Ghoulification has been treated pretty inconsistently throughout the franchise. so a lot of the blanks are left for you to fill in as you please. The process has been depicted as anywhere from near-instantaneous (Moira Brown, the ghoul private at Camp Searchlight in New Vegas) to taking weeks or months. Not everyone even has the potential to become a ghoul, with there necessarily being some sort of selective (possibly genetic) factor that determines whether you die of radiation poisoning or get to live forever as a particularly lively scarecrow, though the mass ghoulification of the Vault 12 residents at Necropolis seems to complicate this question. In the end, the only real consistency with canon would probably be to make sure that you keep things inconsistent, with each individual that actually ghoulifies doing so under circumstances particular to their genetics and their level and duration of radiation exposure.
As to going feral, that's also a big question mark, though it seems to be a result of neurological damage or degradation: lone ghouls or isolated groups undergoing a sort of senility as they lose more and more of what makes them human, intensely traumatized individuals manifesting something similar to an extreme schizoaffective or psychotic aggressive condition, or-- seemingly common among those transformed quickly by intensely high background counts or close proximity to flashpoint events-- the heavy neurological damage cased by ionizing radiation before/during the change (while their unchanged human tissues are still susceptible to radiation damage), and in many cases two or even all three of these factors will apply in varying degrees. As with the transformation itself, it can be a bit of a crap shoot. In all cases, though, the subject is so irreparably damaged that they're reduced almost entirely to their basal instincts, and even those are inconsistent.
It would seem, as borne out in the games without being contradicted or explicitly confirmed, that the process of going feral only occurs when the mind or physical brain of a ghoul degenerates enough that they "lose themselves" and regress to reliance on their instinctual function. These conditions are most likely to be met in ghouls transformed by an extremely damaging amount of radiation over a very short period of time, as opposed to by gradual accumulation in their system.
The only likely ways it would happen to "normal" ghouls whose minds survived the initial transformation would be through long-term "disuse" of their minds, as seen in personality degradation resulting from senility, dementia, or severe dissociative/psychosis-inducing mental states; traumatic brain injury; or through starvation/deprivation. Ghouls need nutrition just as well as intact humans, and long-term malnutrition can have severely damaging effects on the brain. Neurons don't seem to enjoy the same hardiness and rad-regeneration as a ghoul's other tissues (or at least, not nearly at the same level of effectiveness), and over a lot of lean and unsuccessful years (or one or more shorter spans of extreme deprivation), a functionally immortal creature is eventually liable to rack up a lot of brain damage that way.
(Incidentally, I'd bet dollars to donuts that a lot of the ferals you see in the games went this route-- slowly, slipping away in the weeks and months after the bomb, when the world was a nuclear hell incapable of supporting them. Instead of starving to death, they starved to death-of-self, as it were, and got to watch any surviving friends and loved ones do the same. It's a hell of a sorry way to go, and it adds a lot of poignancy to these otherwise throwaway shamblers.)
(Sorry for the lack of brevity here-- it's been a long time since I've done a lore post and this one was done off-the-cuff and coming out from under some pretty heavy anesthesia. I obviously can't source a lot of this, since it comes from extrapolation, but what flesh I've hung from the skeleton set up by canon is born of real-world science and long careful observation of the Fallout world.)
