Could you tell me how long should I change THE ATF FLUID?

and how much ATF I'll need？

MY CAR IS OCTAVIA SLX2.0

Usually about every 2 years or 40000 miles and the capacity of the system should be in your handbook.

The Octavia Service Schedule states that the automatic gearbox ATF level should be inspected every 60,000 km. The oil level of the automatic gearbox final drive should be inspected at the same interval and topped up if necessary. In this thread big K pointed out that specialist tools are needed to check the ATF oil level.