Transmission fluid was originally designed to "last the life of the car" what ever that means. If you are not hard on the transmission, do not do a lot of stop and go driving/towing/racing etc. then you can get away with not having to change out the fluid. When trans fluid heats up it allows the clutch packs to slip/burn and it will leave a residue on everything leading to failure. This heating up also changes the chemcal composition of the fluid so it doesn't work as intended. So if you know you are hard on a transmission you really should be changing out the fluid in the 50,000km/30,000mile range.
