It depends on the size of your VM's. How huge is huge?!?!?
The fastest way to backup a VM is via VCB. I assume we are talking about file or application servers as scripts need to be made to do DB backups to get the databases consistent.
As a rough guide (getting the backup off to tape or disk not the staging area) you should be getting at least 1gb a min on your VMs depending on your setup but I generally get more towards 1.5 - 2gb a min depending on load on the proxy/media server.