Vehicle manufacturers generally recommend that automatic transmission fluid should be changed every 20,000 to 25,000 miles (32,186Km - 40,233Km). This is a basic repair service usually carried out by individual car-owners, and can be more professionally done at service stations worldwide.
Notice: Automatic transmission fluid works in lubricating the torque converters and gears involved in the process of transmission of power from the engine to the wheels. The automatic transmission fluid will need to get replaced when it loses it's viscosity overtime, as it routinely gets subjected to high temperatures of 175 degrees Fahrenheit and above.
A light detail of the inner workings of the transmission process reveal that as the pressure changes in the fluid, the transmission also switches gears accordingly, in the course of operating the vehicle.