(f) Certification periods. The State agency must certify each eligible household for a definite period of time. State agencies must assign the longest certification period possible based on the predictability of the household's circumstances. The first month of the certification period will be the first month for which the household is eligible to participate. The certification period cannot exceed 12 months except to accommodate a household's transitional benefit period and as specified in paragraphs (f)(1) and (f)(2) of this section.