Most vehicle manufactures recommend changing your transmission fluid out every 30,000 to 40,000 miles or every 2 years whichever comes first.
I've run vehicles longer than that without changing the transmission fluid or filter and haven't had problems but if your car is under warranty you should always stick to the regular maintenance schedule.
And getting your transmission fluid and filter changed regularly is cheaper than a new transmission or even the hassle and stress of being broke down on the side of the road and needing your vehicle towed.