To assign a definition to create a site level rule go to :
Administration -> User Expiration -> Rules -> Site.
Setting is available only to Administrator role.
When the rule is created on the Site level, a warning popup with OK and Cancel buttons is displayed with the number of users that will be expired after the rule is applied.
Cancel cancels creation of the rule.
It may take few minutes for a server to apply a rule if a user count is high.