For me, it’s like this:
I’d hold the door for anyone. Most of the time, I end up giving the door an extra push as I pass, rather than standing to the side and letting the person behind me pass first. This is because, usually, it’s much too…strange (awkward would be the word, I suppose) to go so far out of your way to hold the door.
However, I will do so if I am the one opening the door and the person is right behind me because then it’s the more sensible thing to do. If a person is old or in some way disabled, I would always hold the door from the side and let them pass first (unless I’m already inside and see them coming behind me–then I try to hold the door open from the inside [standing to the side, inside the room/building] long enough for them to get there and let go of the door at the moment when, at the time the door shuts, they would already be inside).
Also, I don’t mind someone holding a door for me and I don’t expect a “thank you” when I hold the door (although it’s nice), but when you’re going through doors repeatedly and someone keeps holding doors for you from the outside, it becomes uncomfortable (it would also be weird if I had to repeatedly hold doors for someone).
As far as distances go, I’d say about ten to fifteen feet for a walking person, 20 to 25 feet for someone who is running. Sometimes, one might have to slow down a little as he/she approaches the door and open it a bit slower than usual so that the door-holder doesn’t have to be in an awkward situation where both he and the person behind are embarassed.
Once, I was coming from the side and a woman with a package asked me to hold the door for her. I, of course, obliged, but was put in an uncomfortable situation: the door opened toward her, so I would have had to go around her in order to open. Plus, she was standing so close to it, that she would have had to move away for me to open–and it didn’t like she was about to take a single step from her current position. Now I had a choice: I could either go around her to open it, putting me in the aforementioned position, or I could have gone in and held the door from the inside. But this, too, was a bad idea since she had expressly asked me to hold the door for her.
Um, yeah.
Someone could write a whole book about door-holding.