CTM Snapshot Disk Space?

This might be a stupid question but how much disk space do you need to have for the snapshots?

Current system Vista Home Premium (Harddrive is 100 Gig with only 30 gig free).

Is there a particular percentage of the drive required? Obviously the Compact option will minimize the required space but what’s the ratio?

E

Im sure that depends on the contents of the harddrive…

But i could be wrong.