California Code, Government Code - GOV § 21113

(a) A public agency employee who voluntarily reduces his or her worktime for partial service retirement pursuant to Sections 21110 through 21115, and who subsequently returns to a full-time work schedule pursuant to the policy of the public agency employer, shall be ineligible for five years thereafter to again participate pursuant to those sections. The public agency employer may require a participating employee to return to full-time employment only in the event of an emergency requiring his or her full-time service.

(b) A public agency employee who is participating pursuant to Sections 21110 through 21115 in reduced worktime for partial service retirement may: (1) elect only once in each fiscal year to further reduce his or her worktime; (2) elect only once in five years to increase his or her worktime to another less than full-time schedule.