byez,
You will have no issue with the Amsoil ATF protecting well for 48km. I'd be willing to bet it would do well through 48k miles, as well.
I believe under normal conditions, you'd want to change based on mileage. If you're going to store the vehicle for a length of time, you might consider a fluid change (oil first), but if the vehicle is being operated at least once per week, you should be okay. I believe the ATF is subject to becoming acidic over a period of mileage, depending upon your trip distances and type of driving.