From DODI 6000.13 and AR 351-3

Participants of long-term graduate (leading to a master’s or doctoral degree) education and training in a military or civilian institution shall incur an Actice Duty Obligation (ADO) of three times the length of the education or training for the first year, or portion thereof, unless such degree is incidental to the completion of an established residency or fellowship program; i.e., aerospace medicine.   Additional ADO's for participation in excess of one year shall be one-half year for each half year, or portion thereof.