How long does it take for severe back pain to go away?

This depends on the: Cause but on average several weeks but it could be a couple of days to 3 months for a muscular cause to longer due to a disc herniation or fracture. This assumes this is not caused by a tumor, infection or pain referred from another problem like an ulcer. If the pain is not resolving or worsening, see your doctor.

Sometimes it doesn't: Depends on the reason. Any injuries? Multiple child births? Do you also have chronic fatigue, restless sleep, and need to urinate several times a night? Can you lock your knees and touch your palms to the floor? If this seems to be your pattern you should see an osteopath and find out if you have a chronically injured sacroiliac joint and/or joint hypermobility syndrome.

Depends: On what the cause is and what you do about it. You need a proper physical exam by a pain physician, a physiatrist or osteopathic doctor.