We can all blame Jurassic Park for getting our hopes up that we could bring the dinosaurs using amber-preserved DNA. It turns out DNA won't last nearly long enough for that, but it does hang around longer than we previously knew.
By analyzing hundreds of bones of the extinct moa in New Zealand, scientists were able to peg the half-life of DNA at 521 years. The 158 bones were all buried in very similar conditions, but had an age range of between 800 and 6,000 years, creating a stable, comparative set of data where the researchers could track how the DNA changed over time.
The 521 year half-life means that under ideal conditions, every last hunk of DNA would be gone by 6.8 million years, and there would only be enough to be readable at around 1.5 million years. And that's assuming that the bones have been sitting in a nice, -5°C environment, away from water leeching or other effects.
While that puts the 65-million-year-old dinosaur bones out of reach, it does put a great number of other animals within our grasp. There's also the possibility of using it as a dating method, potentially giving us another avenue of finding the age of buried remains. Since the DNA degradation begins at death, it should give us yet another decay-based way of judging ages. However, before that can begin, we're going to need an immense amount of additional data.
In a report by Nature, only 38.6% of the variation in DNA degradation between samples was due to age — the rest comes from environmental factors. Without being able to account for that, using DNA to absolutely date something would be fraught with inaccuracies.