Time required for transferring recorded file

[anulal 0]

I have a 16 Ch DVR which is recording all the 16 channels 24 hours , the whole recorded files comes to about 20 GB per day .The DVR has USB backup, for taking backup of one day it takes about 5 hours to transfer it from the internal hard disk to external harddisk connected to the USB drive . Can any one tell me how much time normally DVR takes for transfering the files sizes abt 20 GB ?