In general, if the trans is operating properly and fluid is kept cool and filtered well as in aux cooler and filtration, you can operate with the same fluid for a long, long time, especially a high quality modern synthetic fluid that will tend to shear less in service. I am headed for 200,000 miles on current fluid in a 4L60 transmission. I may change it then or just see how long the unit will last at that point. At 200k miles on the fluid, it will have 330,000 miles on the unit/vehicle. Fluid still looks and smells new at this time. The ATF is in better shape than the rest of the vehicle, which is getting somewhat tired.
Without additional cooling or filtration, I would definitely change average fluids by 50k miles or so, and with high quality synthetic fluids around 100k miles or sooner as a general guideline. Some transmissions are harder on fluid than others, and type of service can make a big difference as well, such as towing, mountains, and hot climates.
Once the fluid looks bad or smells, it was probably past the ideal time to change it. I have seen transmissions fail when the fluid still looked and smelled fine. This is usually due to some type of inherent weakness in a component that breaks and then causes a failure. Sometimes we do great maintenance and still have a unit go out due to a known weakness in design or materials. Then on the other hand some folks just get lucky, like yourself, doing zero maintenance and have it go 200k miles or longer. You never know, but at least we try to stack the cards in our favor by doing good maintenance and worthwhile upgrades, right?
