can we say restore time = 1.5 backup time.
As a minimum, the traditional sysadmin "rule of thumb" is that restores take 2 to 3 times more than the backup time. No way is 1.5 enough. This is of course a "rule of thumb" so, as the wise posters above have said, hardware and system load will change your results a lot. Under some scenarios, I could envisage that reload from cpbackup tarballs could take up to 10x the backup time.
Many of us do rsync backups to cover us in the event of a need for a full restore. We can restore ~35Gb from rsync from another machine in the same DC in a few hours on an idle machine. From memory rsync is an order of magnitude faster than restore from tarball. If I was looking at this, I'd want to rsync to a local disk and then to a remote machine. The data usage for the remote rsync is pretty low as rsync only transfers the changed data. If you're wanting to use this to transfer from an old server to a new server, do the rsync once from the live server, then do another when you do the move - in the same DC a re-rsync of only the changed data takes 15 mins, over about 35Gb of user files.
Check for our script "cpmr" which I'll re-post soon, which does the remote rsync, taking just cpanel and the home dirs, not the OS itself.
BTW, mostly unrelated and hopefully not annoyingly, if you just want to restore a single file check out
my cprevert post here from earlier.
Hope this helps...