An individual’s eligibility for a benefit under this subchapter for a month shall be determined on the basis of the individual’s (and eligible spouse’s, if any) income, resources, and other relevant characteristics in such month, and, except as provided in paragraphs (2), (3), (4), (5), and (6), the amount of such benefit shall be determined for such month on the basis of income and other characteristics in the first or, if the Commissioner of Social Security so determines, second month preceding such month. Eligibility for and the amount of such benefits shall be redetermined at such time or times as may be provided by the Commissioner of Social Security.