Most experts suggest a fluid and filter change every 30,000 miles under normal conditions unless you have a new vehicle with Dexron III ATF which is good for 100,000 miles. Under "severe" conditions you should change your trans fluid every 15,000 miles. Severe use being defined as more than 50% use in heavy city traffic with ambient temperatures above 90 degrees. The material the trans housing is made out of also is a factor to consider. Softer metals require more attention.