Devastation is a factor of size of the enemy and length of time they stood there.

I big army will quickly cause a lot of devastation. A small single rebel wont cause any.

How quickly does it go..dont know. Time is the only healer.

Best not let any enemy stand on your land for too long. Also if you capture a city by siege..will you have caused devastation to the area you are about to capture. Better to capture by assault as quickly as possible.