It depends partially on what you are backing up to, a Redundant SIEM, SAN Storage (SSD, Fiber Channel, iSCSI, SATA), NFS or CIFS Share, DAS? Speed of the network / switches backing up over (LAN or WAN)?
We replaced our Primary ESM last year, did a Full Backup to the Redundant, then did a copy to the new / replacement X3, at the time we had over 4TB of data on the ESM itself (did not have to backup / copy the data on the attached DAS as the new ESM would be attached to same).
As I recall it took a little over a day for each copy / backup.
You can use the df - h to monitor the space on your Receiver's.