I see this sort of discussion as two separate questions....

1. how long can I ride it before it breaks?
2. as the fork gets older, the probability of it breaking goes up. At what probability will I decide that it is too dangerous?


For question 1, there is a chance that there might be some external signs that a failure is imminent. With carbon fiber, this is not generally the case. The highest stresses are at the fork crown, where there's not much visibility of any possible flaws and there are large changes in the cross section of the materials, which cause stress risers that contribute to failures. A lot has been learned from those days, leading to larger diameter steerer tubes near the fork crown and smaller stress risers.

2. Manufacturers have much better data on the failure rate of their products than we do. If anyone gets hurt by their products, their losses can be huge. If they find any significant number of forks/frames have failed in the early portion of its lifetime, it is usually cheaper to replace them all than to have to go to court and get fined many millions of dollars.
As consumers, we don't have the benefit of the data regarding fork failure rate. If we know the number of miles and the stresses of those miles (was the rider heavy? were the roads rough and bumpy?), then we have some relative idea of whether the fork's lifetime is used up.
I have one carbon fiber bike, and when I asked the manufacturer what the life of the carbon fork was, they were pretty emphatic in saying that the recommended fork life was defined by the warranty period. I did end up replacing the fork due to some small damage.. which turned out to be due to a design oversight that was corrected on the replacement fork. The bike is 12 years old now, with 30,000 miles. I'm pretty light and the roads are pretty smooth. The risk is relatively small, and since the bike is a recumbent, if the fork breaks, I won't fall onto the ground face first.

If the fork will only be used rarely, then the cumulative risk is fairly small, and it should be fine.
Otherwise, you have to ask yourself how it compares to other risks in your life, and how much you would pay to not fall onto your face at speed.

Also, consider what you pay for insurance for your home and car. Do you expect to get into a car accident, or expect your house to burn down? Probably not, but you pay a fair amount to be insured against those losses. Maybe a new fork is cheap insurance?

Steve in Peoria