It is required once and also reasonable reoccurrent training required. More on FAA drug & alcohol testing regulations – https://www.nationaldrugscreening.com/faa-regulation-49/
Supervisor training (drugs): One-hour of training is required on the specific, contemporaneous physical, behavioral, and performance indicators of probable drug use. In addition, supervisors must receive employee training as defined above.
Supervisor training (alcohol): One-hour of training is required on the physical, behavioral, speech, and performance indicators of probable alcohol misuse.
Reasonable recurrent training is also required.