Principal amounts of all authorized loans shall be repaid in installments over a period of not less than five years (unless sooner repaid) nor more than 10 years beginning not earlier than nine months nor later than one year after the date on which the student ceases to carry at an eligible institution at least one-half the normal full-time academic workload as determined by the institution except:(1) as provided in §21.92(d) of this title (relating to Repayment of Loans); and(2) that the period of the loan may not exceed 15 years from the execution of the note or written agreement evidencing it.