Are you talking about an SSD or HDD? If you have an SSD when you select "Free Space Only" beside Wipe under "Tools" and "Drive Wiper" OR "Options" and then "Settings" you will be advised against using it with the message "Secure wiping of an SSD is NOT recommended". If it is an HDD then the process could take a little while (longer than a few minutes). I can't give you an exact time as it will vary from computer to computer. I use CCleaner on my MacBook Pro and wiping the free space on the second drive (500 GB HDD) can take over 30 minutes..