DNA half lifes
the problem with talking about DNA half lives is that its not relevant here.
If you're talking about radiation, after storage until its half life is up, on average half the useful material is still there.
However with something thats encoded such as DNA, just a tiny change could render the data unreadable. A change of 1% in the data could make the whole set unintelligible.
You'd need to store multiple copies of the data in some kind of DNA raid, but even then without long-term reliability tests you'd be taking a heck of risk. DNA is inherently unstable: until that is overcome, this technology is nothing but an irrelevant curiousity