Earlier quoted context omitted.
Debunked: http://jpkoning.blogspot.com/2019/09/the-life-and-death-of-i...
That article doesn’t “debunk” anything. Did you read it? Their argument is that a currency being “replaced” for “political, economic, and cultural reasons” isn’t a currency failing. What? If they’ve “debunked” the 27 year figure, then what is the correct average lifespan of fiat currencies?
The question is kind of ill-formed in the first place. Take the current US dollar. How long has it been around? Per Wikipedia, it was established by Congress in 1792, which gives it a sedate 230 years of existence. But given that the original analysis is by a gold bug, I suspect the intent is to limit it only to the 51 years that have passed since the end of the gold standard in 1971.
If the goal of the question is to work out how long a fiat currency not backed by gold will last before it's hyperinflated into nothingness, then it stands to reason that the metric which best reaches that conclusion would be in fact the earlier date: the Nixon Shock wasn't an episode of hyperinflation (inflation reached a measly 10-20% annualized). Events like the creation of the euro are even more starkly not relevant to answering that question.
There's also severe methodological issues with completely omitting currencies that haven't failed yet, which ought to be patently obvious. There's another severe bias in terms of the inclusion of currencies on the list, since there's going to be several currencies for which is there is likely insufficient data to establish the lifespan correctly, because records tend not to preserve very well after several centuries.
To attempt to answer your question, I don't have any hard data in front of me. But I would be shocked if the mean lifespan of a currency were substantially less than a century--there are several currencies that lasted several centuries during the Medieval period, such as the ducat lasting ~730 years.