This varies internationally but generally a period of 1-4 years of training and examinations are required.
A specialist is accredited by Membership of the Faculty of Occupational Medicine (MFOM) of The Royal College of Physicians of London. This follows 4 years of accredited training in occupational medicine and multiple examinations.
Details are available from the American College of Occupational and Environmental Medicine Fellowship. It appears that 1-2 years of occupational medicine training is expected.
Australasian training in Occupational and Environmental Medicine requires 3.5 years of full-time equivalent (FTE) training.
Specialist accreditation is open to internists/public health physicians who undertake 1-2 years of training in occupational medicine and a written examination, administered by the Royal College of Physicians and Surgeons of Canada.