At the end of the article it says they thought 1 million was the limit.

> Exactly how far back in time researchers will be able to see remains an open question. “Probably we are close to the limit, but who knows,” says Tyler Murchie, a postdoctoral fellow at McMaster University who develops methods for studying ancient DNA. He notes that the Dutch researchers were successful in combining several techniques to “create a robust reconstruction of this ecosystem.”

> Willerslev once predicted it would be impossible to recover DNA from anything that lived more than a million years ago. Now that he’s broken the record, he is reluctant to say where the limit lies. “I wouldn’t be surprised if...we could go back twice as far,” he says. “But I wouldn't guarantee it.”

Also what it was preserved in also makes a difference so I’d guess 500 years for any normal sample & longer for ones in specific circumstances but I’m just some guy making a wild guess.

> The Danish team says the DNA they found was preserved by freezing temperatures and because it was bound to clay and quartz, which also slows down the process of degradation.