For DNA separation, -20℃ is recommended to keep samples for a long time. Whereas, lower temperature (e.g. -80℃) is required to store RNA, since it is highly sensitive to the RNAase in the environment.  Thus, in case of nucleic acids' degradation, -80℃ is a good choice during your sample collection.

@Xiong Ding thank you very much

-20 centigrade for long time  

I think the -80℃ is a currect  temp.during sample collection.

i agree with Dr. Noaman

-20 is enough somehow but -80 is the best 

-20