This Knowledgebase article provides information about how to calculate the initial file synchronization time for a Neverfail server pair.
The following formula can be used to calculate the initial file synchronization time:
T = a * N + b * M
T = synchronization time in seconds
N = number of files
M = number of Megabytes to be synchronized
a = .026
b = 0.19
The above formula is used to calculate the time it takes to run the initial file synchronization when the files are NOT on the passive server yet.
When the files do exist on the passive server, the constant 'a' does not really change, but the constant 'b' will be lower. It is necessary to determine how much lower the constant 'b' should be.
Note: For small files (say less than 50KB), the per-file overhead (a * N) outweighs the per-Megabyte cost (b * M). This means that there is no benefit realized by copying files across with Robocopy and then verifying the results.
We also have not determined the effect that replication traffic has on the throughput.
The constants 'a' and 'b' are rough guides based on measurements with standard 4.6 software and will vary on different server hardware.
So where N = 18 million and M = 1.2 million MB (i.e. 1.2 TB): T = 500,000 + 200,000 seconds = 700,000 seconds or about 8 days on an empty server.