Ageing and death are linked to the metabolism of a lifeform. A slow metabolism often equals high longevity, and a rapid metabolism equals a short lifespan. There are a lot of different factors though, so take this with a grain of salt.
Fluffies do have a high metabolism, in most headcanons:
- high amounts of food consumed, poop every few hours
- rapid gestation and growth (around 3 to 6 weeks of pregnancy, four to twenty weeks till adulthood in most headcanons)
So it’s save to say they have a rather short lifespan. But how short exactly?
You don’t find that many headcanons that state the actual life expectancy of fluffies, probably because most creators don’t want to set their “maximum age” in stone, or because they didn’t consider it (depending on which side of the fandom you’re examining, most fluffies won’t reach an old age anyway). But what I’ve seen so far is that fluffies can reach an age of around 6 to 10 years (that would also be my headcanon). That’s also a range prominently featured in the Rosemary series.