Saying that is the same as saying “this is the frequency of the sound in seconds”; it doesn’t make sense. Frequency (of sound, but also of other things) is measured in Hertz (1/time) and not in time. Maybe I’m being a bit pedantic about this, but in my experience getting the units of measurements right helps a lot in software development and the understanding of said software by end users.
@mpalmer’s suggestion of “The period of time between backups” looks fine to me; it could be shortened to just “Days between backups”.
As a new user, it was unclear what the settings meant. I had to read the description, and then the next setting and figure out how the two work together. It wasn’t completely clear until I read your post about automated backups.