The simple answer to this question is that security cameras don’t have a mind of their own. They cannot delete or overwrite any footage themselves. Even if the footage is deleted, it will always be on the hard drive.  However, if the footage is manually overwritten, then it will cease to exist. It is advised to keep footage on your camera (if the storage allows it) in case there's a need for it in the future. Generally, most security cameras can keep footage from anywhere between 30 to 90 days.