This can vary depending on driving conditions. Leons recommends replacing the fluid and filter in your automatic transmission every 12,000 miles.
Heat is the number one cause of failures in transmissions.
The transmission oil is critical for lubricating the transmission and reducing friction. When the fluid breaks down and loses its viscosity, it no longer effectively lubricates the transmission. This causes premature and excessive wear and results in transmission failure. The local climate greatly affects the mechanical operation and your vehicle can be forced to work harder when necessary. Temperatures in Southern California are high throughout most of the year.