e.g. someone sets up a wikipage with some content. Then he could enter a time (maybe 6 months) when a mail is generated and informs him to review this page to look if it's content is still up to date.

It should also be possible for an admin to set this option as standard.

I know there's an observing option, but that's not what i think of, cause with that you only get mails when the page is edited.