I have been performaing full erase write zeros using WD Data Lifeguard on several 1.5TB WD drives.
Sometimes it reports 5Hr to completion and the actual time is close.
More often than not it reports 24 to 300 Hrs for the same operation.
I am connecting the disks thru an esata cable and a one of those red ‘manhattan’ external drive bays
These disks have had maybe 10,000 hrs powered on but have been unused for 2-3 years.