Honestly I hate dealing with shops, and I don't trust those flushing machines that recycle as they dump back in contaminants.
So although dropping the pan and replacing the filter if it has one, closing it up and then adding fluid back, only changes about 1/2 -2/3 of the oil depending on the vehicle, because it doesn't drain the torque converter or valve body, i do it yearly, and then at least a bunch is fresh.
If a manual transmission i do it usually every second year since it drains most of it, and no filter, so just pull the drain plug out, go do something for an hour, come back and put plug back in and refill.
I am notorious for buying parts and fluids wherever, but when it comes to transmissions I don't, i go to the dealership and buy the expensive oem fluid so its not mixing it with whatever was already in it.