Replication bandwidth estimation has always been a challenge, because it depends on multiple factors such as the number and size of VMs, change rate (at least daily, per RPO cycle is ideal), RPO target, replication window. Full information about these factors, however, is rarely at hand. You may try to set up a backup job having the same settings as the replication job and test the bandwidth (as the backup job will transfer the same amount of data as the replication job). Veeam ONE (specifically Infrastructure Assessment report packs) may help with estimating change rates and collecting other information about the infrastructure.