This is an important question, with a very simple answer: as long as the patient needs it.
Surgeons all too often think their patients do not have any pain, and if they have, it does not last longer than a day. If only they would ask their patients! Major surgery (chest, abdomen, joint replacements) is very painful for two to four days, and the appropriate pain relief must be provided for this duration of time as well.
Read more:
Pain control
What can I do about period pains and PMS?
Links:
Arthritis Foundation of South Africa
Multiple Sclerosis South Africa
The South African Society of Physiotherapy