I live in the south, so the answer is not clear and simple, but, generally 12-15. Beyond that and you're being flamboyantly over-polite, and actually letting all of the heat or air conditioning out. Less than that and you're slamming a door in someone's face, which is just plain assholery.
edit: incidentally, those vague ethics are gender-independent in this area. Except for the recent influx of asshole yankees, who always slam the door in your face and do their best to cut you off in traffic.
It's a rarity when a Southern male doesn't hold a door for someone, unless we didn't see them coming towards the door. And if there are two sets of doors then it's expected that the first male through the initial door is to hold the next door until the male who was holding the other door has gone through the second door. Yankee males don't understand this system and tend to just fling open the second door for themselves and not hold it for anyone else.
Just enough to make them run awkwardly
Surprisingly, this is the correct answer.
FYI: Men are generally uncomfortable when you open the bathroom stall door for them and say "after you!"..