Transmission fluid as well as the transmission filter in your automatic transmission should be changed every 30,000 miles to 60,000 miles.
If you drive 12,000 miles in a year then that would mean you would need to change your automatic transmission fluid every 3 to 4 years.
I change my transmission fluid after every 5 years regardless of miles because a transmission filter and transmission fluid is much cheaper than a new transmission.
Keeping your transmission fluid changed out often enough and your transmission filter means that your transmission will operate more smoothly and last longer.
My truck has 300,000 miles on the original transmission and engine and the transmission fluid has always been changed out along with the transmission filter as well.