How long should anti-psychiatric drugs like haldol (haloperidol) be used?

Antipsychotic?: I think you probably mean "antipsychotic drugs". These medicines can potentially be used for several years. This depends on whether the medicine is still needed and whether it is being well tolerated.

Diagnosis: If you have a diagnosis of chronic schizophrenia usually life long, depends on the diagnosis and prognosis.

depends on condition: Excellent question. Think about psychiatry as a part of the field of general medicine--how long do most people take their diabetes medicines? Their blood pressure medicines? Their arthritis medicines? For chronic general medical conditions, they generally are life long, and the treatments are generally life long. If you have a life long illness, stay well for life. Stay on your meds. Good luck.