how often should you change transmission fluid
Most manufactuerers say the automatic transmission fluid will last over 100,000 miles with average use. But severe service rquires fluid changes every two years or 24,000 miles. Severe service can mean extended high speed driving, trailer towing, or just driving normally in cold weather. Some manufactureres (like Subaru) state that under severe  conditions you should change the AT fluid and filter every 15,000 miles. If the fluid is bright red and smells sweet, it is pobably still usable. If the fluid color is dark red or brown and smells burnt, change it.