Under severe use, like MM driving, i'd suggest changing ATF at 30k km or less. I change at 20k km and already see a difference in color- from bright red to reddish brown.
Changing ATF in trans is simpler than changing engine oil. Just drain thru drain plug on the A/T oil pan, add ATF thru dipstick fill hole. Usually 3L will suffice, but check level while idling.
So many brands in different grades- there's dexron 2 or 3, Mercon. Find out what grade is recommended by your car's manufacturer.
The owners manual of our cars usually put it at 40K... with bad road conditions and the weather we have (rains, humidity, etc), its probably better to change every 20-30K more or less depending on the drivng conditions you encounter.
Pag nalubog din yung car mo sa baha, its also nice to change after the rains just to be sure na walang water contamination.