Holding the Door

When you walk into a building, when is it appropriate to hold the door for someone? Of course if the person is directly behind you, you hold the door for them. How far away do they have to be before you decide to just close the door? 10 feet? 20 feet? I don't know what the right answer is. What if you are far behind and you realize someone is holding the door for you? Do you keep walking at the same pace? Do you run a little bit to keep them from waiting? It's crazy how much thought goes into such a silly topic like this. You have to make a split second decision too. It's not like you can hold the door for a little bit and then decide they are too far back and let go. You have already committed yourself. Think about that the next time you hold the door for someone.