ATF, or automatic transmission fluid, is one of the most important fluids under your hood. It is the lifeblood of your transmission and takes a beating!
Heat is the biggest enemy to this fluid because there is a lot of friction and pressure generated within the transmission. The byproduct of all these forces is heat. In every automatic car, there is a small cooler that helps to cool the circulated fluid before it returns to the transmission. Even with this cooling, the ATF breaks down over time and loses its ability to properly lubricate.
Some manufacturers have a 30k interval, some are 60k and some are even 'lifetime'. The only real way to determine the condition of the ATF is simply look at it. It's a judgement call and pretty easy to see for yourself.
Rest assured, there is no such thing as a lifetime fluid in any car.