Carbon is also prone to material fatigue. Its the same as with windsurfing masts - they all brake eventually, but its really hard to predict when it will happen. One might last for two years and another one can go for 5 or 6, maybe even more. It also depends on how much you use it. I think the same goes with fins, so you can never be certain when buying a used fin.
Anyway, as I mentioned before, Virus fins, I think, are pretty cheap brand new. Boss fins are also cheaper than other popular ones, so maybe its better to buy a new one and have warranty for at least a year.