Redetermination of eligibility shall be made by the local department or service vendor: When required on the basis of information the agency has obtained about anticipated changes in the individual's situation. Promptly, not to exceed 30 days, after information is obtained by the local department of social services or a purchase of service vendor about changes which have occurred in the individual's circumstances that may make him ineligible. Periodically, but not less frequently than every 6 months. The need for service shall be redetermined at least every 6 months. For individuals whose monthly gross income at the time of determination is derived exclusively from pensions or Social Security benefits, or SSI benefits or a combination thereof, redetermination may be made of their income eligibility at 12-month intervals. Recipients of services for which eligibility is determined on a group basis are not subject to the redetermination requirements.