The payments provided for by sections 79-993, 79-994, and 79-997 may be made in equal installments over a period of not to exceed two years from the date of the election to make such payments. The payments provided for by section 79-995 may be made in equal installments over a period of not to exceed three years from the date of election to make such payments. Any person who elects to make payments on an installment basis shall be credited with prior service only in six-month increments and only after payment has been made to the retirement system to purchase each additional six-month increment.