I changed mine at 25 k (fluid only, using a oral syringe instead of turkey baster).
It was really dirty as u can see in photo. I changed because I thought there was noise coming from the power steering and the serpentine belt. (Unrelated: Turns out it is a normal sound from alternator with 328 design)
There is a school of thought that almost as soon as you replace the fluid it will quickly get dirty.
I might check on the fluid soon, now at 29 k. If its just as dirty as it was at 25 k, I might agree with this school of thought and do it only for paranoid reasons every 30 k, but not worry if it doesn't get done.