It does seem, from varios posts, that the batteries last just over 2 years irrespective of how they are used. Also the warnings seem to be hit and miss.
I would suggest that they are changed every 24 months even if they seem to be OK.
It very easy to do. Open the key and put something like a big screwdriver blade or even the blunt end of a dinner knife or fork in the slot where the key rests and twist. Don’t try and do it with your fingers as the plastic edges are sharp and you may cut yourself. The cover will pop off easily. There is a very small slot, just big enough for a very small screwdriver, in the area surrounding the battery. Use that to release the battery. Inserting the new one is easy, just pop it in and press into place.
Faults and Technical chat for the Seat Ateca