You may store cDNA at -20°C for several years but avoid freeze-thaw cycles. If you are going to use the cDNA for several experiments over time freeze appropriate aliquots.

Purified transcripted DNA can be stored at -20 degree Celsius for short period of time and at -70 degree Celsius for long period of time.

https://www.qiagen.com/at/resources/download.aspx?id=f0de5533-3dd1-4835-8820-1f5c088dd800&lang=en

Store at -20 C in 70 % EtOH, it will last forever. Just make sure the tube is well taped (e.g. with Parafilm) so that EtOH does not evaporate. When you want to use it again, spin it down, dry out, and dissolve in water/buffer.

At -80°C