I wonder how many generations this kind of story can be passed though and still retain the distinguishing characteristics?
IMHO it all depends on how loosely / strictly the given cultural group (in this case the Apa Tani) manage their oral histories.
If there's a strong tradition of designated storytellers / oral historians whom are drilled on a coherent / canonical body of lore, the reasonably retrospective horizon could conceivably extend back centuries.
If there's little or no such attention given to managing oral history, it's anybody's guess how long past - and how reliable - the hand-me-down lore may be.
If the buru was such a monitor lizard and it died out (say) 4-500 years ago, folk memories might persist and in marshy land, often acidic, bones would be scarce and buried.
I don't know what sort of timeframe applies in this case, because I've never seen any claims as to how long the Apa Tani have occupied that particular valley. For all I know there's a solid answer to this question, but I don't know that any of the crypto investigators ever made the effort to find out.
Yes, I realize any such remains might be few and require effort to locate. I never claimed it would be easy. My point was simply that tangible cryptids should leave tangible evidence, and I don't recall any of the accounts / reports mentioning efforts to locate past remains.
I've not paid a lot of attention to the buru story because I see it as subject to a double stigma.
Both the historical details of Apa Tani settlement
and some form of recent / contemporary evidence would be required to elevate the buru to the level of, say, Nessie or Bigfoot. These (and other ... ) more widely known cryptids are recognized (or at least purported ... ) based on substantially more than someone's handed-down account of an arbitrarily distant past.
These more popular cryptid stories enjoy the benefit of ongoing sightings to keep them alive. Given the obvious lack of such ongoing sightings for an extinct cryptid, the best one could hope for would be discovery of remains.