Usually DNA sample manipulation is realized with an ice box at hand in order to avoid degradation, and also its storage is set at -20ºC. Nevertheless, DNA has been obtained from really ancient samples that were not in a cold environment and even our DNA is at 36ºC. Furthermore, when performing PCR, samples are put into a massive heat shock and at high temperatures. Then how needed are those protocols in order to grant correct DNA quality? how much can a sample be left without need to worry? and how does this degradation happen (DNAse free environment)
DNA decomposition isn't a phenomenon that is too common. In nearly all cases, DNA is broken apart to the accidental contamination with nucleases. Long-term storage of DNA in the -20 is also suggested in order to significantly lower the rates of these nuclease enzymes.
Additionally, DNA only truly fragments at very high temperatures (>90-100 degrees Celsius) due to the nature of the covalent chemical bonds that form DNA. Nearly any organic molecule will degrade at very high temperatures, and some will degrade at lower temperatures due to unstable structures or heat-labile bonds.
Of course DNA will deanneal at somewhat-high temperatures due to its double standedness , but allows for reannealing to the initial molecule once the temperature is brought down.
One additional reason for potential DNA degradation is DNA shearing, which will occurs when you tear up your DNA physically due to exposure to rigorous shaking, resuspending or other "violent" techniques.