I am asked this question almost daily by customers. To give you some insight as to how the transmission works, an automatic transmission creates a lot of internal heat through friction. The friction of the fluid churning inside the torque converter which bolts up to the back of the engine, friction is created when the clutch plates in the torque converter engage and the normal friction created by the gears and the bearings are carrying their loads. It doesn´t take long for the automatic transmission fluid (ATF) to heat up once the vehicle is in motion. Normal driving will raise fluid temperatures to 175 degrees F which is the usual temperature range at which most fluids are designed to operate. If the fluid temperatures can be held at 175 degrees F., ATF will last almost indefinitely, up to about 100,000 miles. But if the fluid temperature goes much higher, the life of the fluid begins to plummet. The problem is even normal driving can push fluid temperature well beyond safe limits. And once that happens, the trouble begins. As a rule of thumb, every 20 degree increase in operating temperature above 175 degree F, cuts the life of the fluid in half. If the fluid temperature reaches 195 degree F for instance, fluid life is reduced to 50,000 miles. If it reaches 220 degree, then the fluids life is about 25,000 miles. As you can see, depending on the type of driving you do or how you drive will depend on how often you should change your transmission fluid or how soon your transmission will fail. You will need to analyze your driving in order to determine when to change your fluid. As I tell many customers, changing your oil is not bad on your components, only if you wait to long to do so. With the high costs of transmissions these days, you could be well into thousands of dollars for a replacement, so a few dollars now, will save you thousands later.