The length of residency depends mostly on the field a graduate chooses to take. Medical specialties such as family medicine and internal medicine often require only three years, whereas surgery usually requires a minimum of five. Subspecialization (vascular or orthopedic spine surgery as a branch of surgery, for example) in any field will add time to post-graduate training.
For more information on specific medical residency programs, see the American Medical Association's Fellowship and Residency Electronic Interactive Database.
Dental residencies for general practice, known as GPRs, are generally one year, with a possibility of a second year at some facilities. Dental specialties, such as Oral and maxillofacial surgery also require further training (4-6 years). Most specialty programs do not require students to have completed a year of GPR, but it most definitely increases the chances of the applicant.
Pharmacy residencies are usually one year, but a PGY-2 can be completed, often as an option, for pharmacy specialties such as critical care, cardiology, oncology, etc.
In some teaching institutions, trainees are required to indicate level of training on all signatures (John Doe, M.D., PGY-1 or R-1).
Residencies are also offered for those in the Physician Associate (assistant) profession in a variety of specialties such as surgery and emergency medicine. Controvery exists as to whether this should be mandatory for subspecialty work.