An assessment lease--
(a) takes effect on the date on which it is granted or on such later date, or on the occurrence of such later event, as the decision-maker may determine, and
(b) ceases to have effect on the expiration of--(i) 2 years after the date on which it took effect, in the case of an assessment (mineral owner) lease, or(ii) such period (not exceeding 6 years) as the decision-maker determines, in the case of any other assessment lease.