In the past, 30 k miles was about right for my trans changes. I never worried about doing it too often because one only gets about 1/2 to 2/3 of the fluid out. If the fluid comes out brown, then you are still leaving bad fluid in the trans.
Yes, one can pull off a cooler line and bleed off some more bad fluid but you won't get it all.
So, best to change it a wee bit more often than needed, IMHO.