So, the Tundra has a sealed transmission. That means that the fluid is not allowed to freely interact with air. The oxygen in air degrades the fluid over time and it has been known since the 1950's that if you seal transmission fluid from air, it is not degraded by the oxygen. The air that is in the transmission when assembled does interact with the fluid until the oxygen is used up and you are left with an inert atmosphere of nitrogen that protects the fluid.
In a sealed transmission, if you don't tow heavy, off road your truck, and drive in such a way that you minimize shifts and keep the torque converter locked up, your transmission fluid will last a lifetime, which probably means several hundred thousands of miles.
However, if you tow heavy, drive your truck so it is constantly shifting or the torque converter is running unlocked a lot (ie, around town driving, off-roading, etc.), the fluid is being stressed through the torque converter and will wear over time and need to be replaced. In these situations, you will need to replace your fluid.
It is normal for transmission fluid to turn maroon in color. But when it turns brown or black, that is bad. The brown/black is caused by the transmission fluid getting burnt from heat and stress and indicates that you definitely need to have the fluid changed and may have done damage to your transmission.
Personally, I only occasionally tow a light travel trailer and when I do, I keep the torque converter locked up and drive it easy. When I am not towing, I basically and driving long distance on highways with little to no stress on the transmission. In my case, I think a drop the pan, replace the filter and fluid in the pan, at about 100-150K interval is sufficient for my transmission fluid to last several hundred thousand miles (and that is what I have done so far and am at 180K and no issues).
But if you are in town driving, towing heavy and hard, off-roading, etc, you should change your fluid more often and consider a full fluid change instead of just replacing what is in the pan.
Last, you won't hurt anything by changing the fluid more often than needed. It just costs money. But you can damage a sealed transmission if you don't change it right and don't get the level right, so make sure you know what you are doing or that your mechanic knows what they are doing. It is not an easy process. I had mine done at the dealership and the filter/pan fluid change was under $200. I am sure a full fluid change is probably double that price.