Edit: Wow a lot of bs going on in the comments. First of all, there's an immense number of ways a DNA strand can degrade, and only one of them splits the strand in two. The relative importance of all these pathways depend on the environment of the DNA, which obviously changes for each and every fossil you have. The global kinetics of "DNA degradation" are supposedly first-order, which implies constant half-life.
Edit: Wow a lot of bs going on in the comments. First of all, there's an immense number of ways a DNA strand can degrade, and only one of them splits the strand in two. The relative importance of all these pathways depend on the environment of the DNA, which obviously changes for each and every fossil you have. The global kinetics of "DNA degradation" are supposedly first-order, which implies constant half-life.
http://rspb.royalsocietypublishing.org/content/279/1748/4724