Depends absolutely on the canon because so many people want Fluffies to be both ever-present in unlimited numbers literally everywhere and yet 100% guaranteed to die and take any offspring with them before there can ever be another generation.
I figure Fluffies were developed poorly in a manner similar to something like World Of Warcraft where teams were let loose with resources and half the time they wound up doing the same thing different ways while nobody was working on other parts at all. Different physical designs. Different programming builds. All mixed, becoming different breeds.
Factor in natural selection, intentional breeding done by humans, and knockoffs plus attempts to alter them, and you get the modern Fluffy which is alike sometimes only in their ability to create offspring together with varying degrees of survival at birth and defects.
Some breeds are prized in spite, or because of, their ability to snap their own neck on a sneeze, puke while swallowing and rupture their throat, or giggle themselves to death. Others are just basically badgers that look a bit like fat short horses and can actually outcompete all local species.
I set my canon anout 70 or whatever years after they were released, so the initial breeding separated out survivalist ferals from urban ferals from weak domestics and weaker fancy domestics. Lets say it took maybe ten generations of ferals to select out their most handicapping traits like inability to approach water and desire to cull most of their own offspring.
So ultimately its just a matter of how much ancestry they have that has Gold Plastic Syndrome and otherwise how smart the individual is. With some that have vastly unfair advantages like actual hooves or efficient-ish digestion.