With a 5mg dose of melatonin, its half-life remains approximately 1 to 2 hours, indicating that half of the dose is metabolized and eliminated within this timeframe. Typically, it takes about 4 to 8 hours for most of the melatonin to be cleared from the body after ingestion.With a 5mg dose of melatonin, its half-life remains approximately 1 to 2 hours, indicating that half of the dose is metabolized and eliminated within this timeframe. Typically, it takes about 4 to 8 hours for most of the melatonin to be cleared from the body after ingestion.