I changed the fluid and filter at 55k. How often is it recommended?
According to the owner's manual, for "normal" vehicle use, every 100k miles. For "severe" use, every 50k. But if you look up GM's definition of "normal" and "severe," the vast majority are "severe."
Myself, I do every 50k at a minimum. And I take it to the dealer for a full flush - all 16 quarts. My shortest inverval was 30k, when I took several trips out West with my 8600-lb travel trailer in one summer. It hits those stupid temperatures when climbing the Continental Divide. That pic was from ascending towards the Eisenhower Tunnel, long 7% grade, elevation 11,000 ft.
Trans fluid is easy to maintain. Pull the dipstick, smell it, rub it on your fingers, and put a drop on a piece of white paper towel. If it's bright red, smells hydraulicky/detergenty, and still feels like oil with no grit, it's good. If it's starting to turn brown (or worse), smells burned, or is gritty, replace it.
This thread reminded me to check my fluid in the Suburban. See pic below. The fluid has 33k miles on it. Zero towing over that time, but lots of high-speed (85-100+) cruising. I also splashed on some new fluid for contrast/comparison. It looks/smells/feels as I'd expect for that mileage. Still smells detergenty, nice and smooth between my fingers, lost some of the bright red color. I'm confident letting it go to 50k. Unless a towing trip pops up on my schedule, then I'll flush it before I head out.