Brief Answer:
Duration is variable and depends

Detailed Answer:
Thank you for asking!
Delirium is defined as a transient, usually reversible, cause of cerebral dysfunction and manifests clinically with a wide range of neuropsychiatric abnormalities. It can occur at any age, but it occurs more commonly in patients who are elderly and have compromised mental status.
The duration of it is a purely variable phenomenon and depends on age and cause. eg adults have delirium for shorter periods but elders get it for long. postoperative delirium is for hours but medical illnesses have longer durations. Demented patients have longer than non demented.
Here are some average rough states for the average duration of the delirium.
These are the statistics of a research of NCBI
The rate of disappearance of delirium appeared log linear for approximately 2 weeks, but rate of resolution for medical patients was slower than for postoperative patients. The mean and median duration of delirium for medical patients were, respectively, 13.2 and 8 days, and for postoperative patients, 7.6 days and 6 days. Combined mortality over 3 1/2 years was 46.8%. Demented patients had longer average durations of delirium than nondemented patients, but differences were not statistically significant because of large variance.
These are some of the scales to assess the severity of delirium, the more severe delirium and the more duration.
Confusion Assessment Method (CAM)
Delirium Symptom Interview (DSI)
Confusion Assessment Method for the Intensive Care Unit (CAM-ICU)
Intensive Care Delirium Screening Checklist (ICDSC)
Delirium symptom severity can be assessed by the Delirium Detection Scale (DDS) and the Memorial Delirium Assessment Scale (MDAS).
The goal of treatment is to determine the cause of the delirium and stop or reverse it. Components of delirium management include supportive therapy and pharmacologic management.

Fluid and nutrition should be given carefully because the patient may be unwilling or physically unable to maintain a balanced intake. For the patient suspected of having alcohol toxicity or alcohol withdrawal, therapy should include multivitamins, especially thiamine.

Reorientation techniques or memory cues such as a calendar, clocks, and family photos may be helpful. The environment should be stable, quiet, and well-lighted.

Delirium that causes injury to the patient or others should be treated with medications. The most common medications used are neuroleptics. Benzodiazepines often are used for withdrawal states.
I hope it helps. Take good care of yourself and don't forget to close the discussion please.
Regards
S Khan

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