Except as provided in paragraph (b) of this section, a person's eligibility on a register is terminated when:
He accepts a career or career-conditional appointment from the register; or
OPM terminates the eligibility of all persons on the register.
OPM may determine that in particular types of cases eligibility may not be terminated in less than 1 year. OPM shall publish the conditions under which eligibility may not be terminated in less than 1 year.