At what mileage should transmission fluid be replaced ?

We actually perform Automatic Transmission fluid servicing on Honda cars every 15,000 miles to prevent it from getting dirty. There is no replaceable filter in those transmissions, so keeping it clean, will keep it healthy, and like the answer above, it is relatively inexpensive to service, especially if kept clean. Dave Toole Toole's Garage

Whenever it gets dirty. Honda dealers differ, but most professional agree that 30-50,000 miles for the initial drain, then inspect every oil change after that. Your owners manual will give a specification, but we have noticed in the field, that an inexpensive transmission service every couple of years will add significant life to the car.