Changing the transmission fluid in a vehicle should take place based on the manufacturer's written recommendation, which ranges from 30,000 to 60,000 miles for manual transmissions and 60,000 to 100,000 miles for automatic transmissions. More frequent changes do not harm the components.
Like many of the fluids used in modern vehicles, transmission fluid is a petroleum-based liquid designed to act as both a lubricant and a hydraulic fluid. It both assists with gear changes and ensures that gears move freely within the transmission. Every manufacturer is different, and some even recommend a transmission flush every 15,000 miles for a manual gear box. Alternately, some automatic transmissions have no written recommendations at all.