Treas. Reg. §1.401(a)(9)-6, A-3
Q-3. How long is a period certain under a defined benefit plan permitted to extend?
    A-3. (a) Distributions commencing during the employee's life. The period certain for any annuity distributions commencing during the life of the employee with an annuity starting date on or after the employee's required beginning date generally is not permitted to exceed the applicable distribution period for the employee (determined in accordance with the Uniform Lifetime Table in A-2 of Sec. 1.401(a)(9)-9) for the calendar year that contains the annuity starting date. See A-10 of this section for the rule for annuity payments with an annuity starting date before the required beginning date. However, if the employee's sole beneficiary is the employee's spouse, the period certain is permitted to be as long as the joint life and last survivor expectancy of the employee and the employee's spouse, if longer than the applicable distribution period for the employee, provided the period certain is not provided in conjunction with a life annuity under A-1(b) of this section.
    (b) Distributions commencing after the employee's death. (1) If annuity distributions commence after the death of the employee under the life expectancy rule (under section 401(a)(9)(B)(iii) or (iv)), the period certain for any distributions commencing after death cannot exceed the applicable distribution period determined under A-5(b) of Sec. 1.401(a)(9)-5 for the distribution calendar year that contains the annuity starting date.
    (2) If the annuity starting date is in a calendar year before the first distribution calendar year, the period certain may not exceed the life expectancy of the designated beneficiary using the beneficiary's age in the year that contains the annuity starting date.