In most cases, isotopes can be detected for up to about ten half-lives, after which prevalence is reduce to 1/1,000th the initial concentration. In this case, that would be about 240,000 to 650,000 years.

For comparison, h sapiens seems to have diverged from its common ancestor to the other great apes about 2 million years ago, and achieved modern anatomical form about 200,000 years ago.

The period 240kya -- 650kya corresponds roughly to h. heidelbergensis.

https://en.wikipedia.org/wiki/Homo_heidelbergensis

The well-known organic-material radiodating isotope carbon-14 has a half-life of about 5,730 years, and is effective in dating materials to about 60,000 years ago.

We're going to need a bigger boat ^W^W longer half life.

Probably on the order of 200,000 to 10 million years.

Daughter / decay elements of plutonium don't indicate the initial quantity, and might (I'm not sure of this) also be the result of other decay chains.

Radiodating used to determine the age of the Earth relies on uranium-lead dating using samples found within zircon crystals. The crystals cannot form in the presence of lead, so any lead found within a crystal must have appeared there after the crystal formed. As both the mother uranium and daughter lead are within the same sample, the ratio can be clearly determined.

https://en.wikipedia.org/wiki/Uranium%E2%80%93lead_dating

Absent a similar containment for plutonium in the formations being described in TFA, that may not be possible.

For C-14 dating, the ratio of carbon isotropes within living tissue is known. The residual C-14 is what's left after the original organism has died, and the ratio then determines age. Given the short timespans afforded by C-14 dating, concerns with contamination or loss are lower.