13. A person transferred to the reserve in consequence of the exercise of the rights conferred by regulations 4, 5, 6 and 7 or under regulation 9 with the approval of the competent air force authority shall serve in the reserve for a period beginning on the date of his transfer and ending—
(a)6 years later, or
(b)22 years after the relevant date,
whichever is the sooner or for such shorter period as the competent air force authority may approve.