This is what I found:
AIR FORCE INSTRUCTION 36-3212
5.16. Establishment of Disability Separation/Retirement Dates.
5.16.1. AFPC/DPFDD determines the retirement or discharge date at the time the orders are published. Appropriate processing and permissive TDY time are considered when establishing the effective retirement date. All disability separation/retirement dates will be at the end of the month.
5.16.2. Service members retiring or separating for disability may sell their accrued leave up to the maximum of 60 calendar days permitted by law. If leave is not sold, it is used as terminal or ordinary leave through date of separation or retirement per AFI 36-2910.
5.16.3. AFPC/DPFDD will establish disability separation and retirement dates as follows:
5.16.3.1. The date of separation or retirement is established upon finalization of the service member’s case. (T-3). AFPC/DPFDD calculates the effective discharge date providing a minimum of ten calendar days for out-processing, 20 calendar days Permissive Temporary Duty and accrued leave.
5.16.3.2. For the purposes of Benefits Delivery at Discharge and receipt of benefits and compensation from the Department of Veterans Affairs, the date will be established no earlier than three calendar days from the end of the month. Note: Any unsellable leave is required to be taken prior to the service member’s separation or retirement date.