As long as you are storing the DNA at pH 8.0 (e.g., in non-DEPC-treated nuclease-free TE, pH 8.0) it should be safe for a long time (years) at -20C or -80C.  Be sure the tubes are sealed tightly so that sublimation of the buffer is held to a minimum; perhaps wrap the tops with parafilm.

As long as you are storing the DNA at pH 8.0 (e.g., in non-DEPC-treated nuclease-free TE, pH 8.0) it should be safe for a long time (years) at -20C or -80C.  Be sure the tubes are sealed tightly so that sublimation of the buffer is held to a minimum; perhaps wrap the tops with parafilm.

Diana, storage of genomic DNA at pH 8-8.5 with TE buffer is even safe at 4 C for a year or so. But to be on a safe side still many researchers prefer long term storage at -80 C, which is effective for 2 years minimum without a doubt. But I would suggest if you have liquid nitrogen tank facility then try to freeze there, as in vitreous state DNA will be more inert and 100 % safe from chemical degradation or any possible nuclease activity or from any oxidation effect within the sample tubes.

You may also find  that many researchers prefer to store DNA as precipitation in Ethanol at - 80 C, but here you need to re wash and purify your DNA  from ethanol and then successive transfer to adequate buffer before using it for any downstream application later.

Also whenever you save your original gDNA sample it is better to distribute in few aliquots and safe it. What is important to know here that never safe DNA in water for long term storage and what Dr Jack said the proper sealing of  your tubes, which is often neglected. With Regards...

Dear Diana,

You can use your genomic DNA after 2 to 3 years stored on  -20 C. But you should be kept  in mind that the genomic DNA should be stored in TE buffer at pH 8-8.5. Genomic DNA should be kept at -20C and the temperature should be not interrupted.

No problem you use your DNA without any hesitation.

Best Regards

 Kamlesh K Yadav

Can I store genomic DNA at -20 C for a year without degradation that could affect qPCR runs by that time? - ResearchGate. Available from: https://www.researchgate.net/post/Can_I_store_genomic_DNA_at-20_C_for_a_year_without_degradation_that_could_affect_qPCR_runs_by_that_time [accessed Dec 2, 2016].

In fact in certain labs they are routinely storing genomic DNA at -20C and it does not affect QPCR.

I think the two biggest factors would be PH - as discussed above, and then you also need to look at thermostability of the cooling unit. Eg a -20 freezer. If people are using it a lot and taking things in and out etc. Because you would have small DNA volumes which would thaw quickly if for eg someone takes them out often, to move them around to put in their reagents etc. Freeze thaw cycles and temperature fluctuations are actually worse for DNA than a constant temperature. So as long at it is more or less contant, no issue. 

In short - you should be fine. DNA is pretty stable. Its RNA that can be an issue