According to Nature, palaeogeneticists led by Morten Allentoft at the University of Copenhagen and Michael Bunce at Murdoch University in Perth, Australia, looked at the DNA they found in the leg bones of the extinct moa bird and where able to determine the half life of DNA. Reportedly, DNA will stay in one piece for only 521 years.
By comparing the specimens' ages and degrees of DNA degradation, the researchers calculated that DNA has a half-life of 521 years. That means that after 521 years, half of the bonds between nucleotides in the backbone of a sample would have broken; after another 521 years half of the remaining bonds would have gone; and so on.
The team predicts that even in a bone at an ideal preservation temperature of -5 °C, effectively every bond would be destroyed after a maximum of 6.8 million years. The DNA would cease to be readable much earlier -- perhaps after roughly 1.5 million years, when the remaining strands would be too short to give meaningful information.