Certain file types can be excluded from scan, the default configuration exlcudes dll, sys, vxd and inf files from the scan. You could add more files that are important in your case or remove any of the default ones. You could on the other hand limit the scan to files matching a certain filter. It would be easy to setup a scan that would only scan for images for instance using this filter. (good way to check for duplicates in image collections of 100000+ images)

Last but not least you can leave out files that have a certain file attribute such as read only, system or hidden. System files are excluded from scans by default. A last parameter makes it easy to define a rule that files will not be scanned if the file size exceeds or undercuts a give size.

The lenghts of a scan depends on various factors such as hard drive space, filters that have been set and speed of hard drive. Doublekiller is a clever program that makes it easy to scan through a large list of files and free up disk space in the progress.