When do you hold the door for someone?

I encounter a problem every morning when I leave my building, I live in a condo building, how far behind does someone have to be for you to hold the door for them. You don't want to be rude and have the door slam in their face but in our lobby the distance between the elavator and the doors is about 50 feet. When I make eye contact with someone after I open the door for myself then I feel obligated to keep it open for them.

I guess there are a couple of factors that come into play here.

1. Is the location of the door in a place where it is a good chance that you will encounter the same person again? if so then error on the longer side to hold the door
2. Is there more than one person? If there is a group coming and by not holding the door they still have at least 5 feet to the door before it shuts, then don't hold the door
3. If it is someone that has their hands full or has a stroller, then always hold the door as long as it isn't a really long time(over 10 sec) as that would seem awkward
4. Anyone with a dog, because let's be serious, dogs can't open doors
5. There used to be a distinction in regards to holding the door for women vs. guys...that rule gets thrown out, unless it is an elderly woman, then error on the longer side to wait for holding the door

I hope these 5 rules help you look like a proper person...please add your own thoughts if there are any other rules for holding the door.