Hi......
Its advisable to change your automatic transmission fluid once every 20,000 - 30,000km......... in fact, its a must or you will find the gear change in your transmission very rough and jerky. On long run will definitely harm your gearbox and 'crashed' it eventually.
Really, it got to depends on how you drive, and your luck. There's no exact figure on how many km the auto gearbox can last actually. Its suppose to last you donkey years (imagine you buy an auto car in M'sia, which you can drive for 20-30 yrs). A lot depends on your driving habit as well, and if you have change your ATF frequently.
In Singapore context, i will very much dare to say that its okie for the gearbox to last 10 years. Or simply putting a figure, 300,000km is very much possible (with proper care), so do not worry that much. I'm sure as the driver of the car, you can definitely 'feel' something is not right with the transmission when its really going to 'die'

So don't worry abt such things now when it haven't happen......... just enjoy your ride, and happy driving.