Hi there,
There are quite some varieties that would affect the battery life, like what you said, been idle for a while...
and CPU and GPU spec, OS(Win10/Win11), screen brightness, wifi signal, bluetooth, power setting in MyASUS and Windows.
There are many thing you can try to adjust if the battery life can be improve or maintain.
You can also run the battery report to see if the capacity did decline.
ASUS Battery Information Center
Thank you.