I retreat when the enemy catches up (usually when playing against another player) and their army is larger than mine. In this case, it's necessary to retreat; if there's a town portal, that's good, if not, I fight (gotta give him a beating) and then run... but that doesn't happen often.
And with neutral creatures, almost never, unless I forget to send a unit for scouting and get ambushed))) especially when playing against humans – you won't be able to load the game afterwards.