The answer depends on what are you replicating? Do you want to replicate all, or just one of: Polices and Groups, Client Packages and Content Updated (do not recommend over 1Mbps link) and Logs.
Take a look at Best Practice to determine type and replication schedule:
https://support.symantec.com/en_US/article.TECH160642.html
You can also truncate logs before replication.
If you are looking just the time needed to transfer 20GB over 1Mbps link (in theory), it is:
Speed: 1 MB/s
Size: 20 GB
6 Hours, 15 Minutes and 28 Seconds