Snapshot Size?

Now that I've accumulated a number of virtual machines (Windows XP, 7, and 8), I am considering using the "snapshot" feature to speed up backups with Time Machine.

But I am also concerned about filling up my laptop's hard drive with lots of snapshots. Before I start down this path, and then have to fuss with merging or deleting snapshots, can someone give me a rough estimate of the file size for snapshots? For example, a couple of my virtual machines are about 30 GB in size - how big a file would each snapshot be?

Is there a percentage amount, like 2% or 5% or 10% of the original virtual machine, that I can use to estimate the size of snapshots? Or is there some fixed size?

Thanks in advance. ...pt

 

Snapshots (at least with versions 6 & 8) can cause the total disk space utilization to exceed 3x the original space allocation; incident #1563780 was created back in January but Parallels Support was not receptive of the opportunity to re-examine why disk files are taking up way more space than they were with Parallels Desktop 3 (they also said the formula is "(size of hard drive) times (number of snapshots)" but didn't offer to have the developers provide the formula).