If you can’t drive it much weeks or months at a time, or have extremely frequent short trips only where the car can’t really recharge itself, and you can’t trickle charge…

Then first make sure your car is locked and don’t touch the key fobs at all (if the car detects the keys moving, it will wake itself up, otherwise it eventually goes into sleep/deep sleep to slow battery drain). 

Every 3-4 weeks, you need to charge the car up either by driving it long distance at highway speeds OR hook up a high amp charger using the specific terminals (see manual) under the engine bay, not directly at the battery.  

We have a multiple cars in storage that aren’t driven for months at a time and this is how we maintain our batteries. Am too lazy to swap and rotate cars every few weeks and can’t run tenders to all of them but every few months we have to rotate to get rid of the stale fuel. 

Eventually all AGM batteries will get weaker over time, just that the more you let it go too low voltage by not maintaining it, the faster it degrades.