How long can I leave the fridge open?

In terms of food safety you can leave the door open for the same amount of time that your food can safely sit on the counter when taken out of the fridge. This is a couple of hours for most potentially hazardous foods as indicated in the answer to the linked question. This a cumulative time, so if you leave the door open for an hour three days in a row you're going to have to throw out a lot of food.

Of course this isn't really what you meant. If you're just leaving the door open while you pour yourself a glass of milk, the temperature of the food in the fridge isn't likely to reach the danger zone. How long that takes will depend on factors like the nature of the food and its location in the fridge, but I wouldn't worry about if you unless have it open for more than minute or so.

So from a food safety perspective you don't have to worry too much about leaving the door open. From an energy savings perspective you do however want to keep the length of time of the door is open to the minimum. The temperature of the air in the fridge will rise faster than temperature of the food, so every second you have the door open the fridge has do that much more work to cool it down again. This is probably the real reason why your parents told you to close the door immediately.

The answer depends on a lot of factors, but some of the significant factors include:

1. How full your fridge is.
2. What's in your fridge.
3. What the ambient temperature is in the room.
4. What the humidity is in the room.

Basically ... every time you open the door you effectively exchange the air inside the fridge with the room. The longer that you keep the door open, the more air that gets exchanged. To explain the variables above:

1. The less free space in your fridge, the less air there is to be exchanged. (although packing it too tightly means that things can't cool off as efficiently, either).
2. Some items are more sensitive to temperature change ... but more importantly, things have different thermal densities -- a liter of water will 'hold' more cold than will a liter of most dried items.
3. Warmer air means more energy flux when the air's exchanged.
4. Moist air holds more energy at a given temperature than dry air. Too high of humidity and you also get condensation inside the fridge which may cause other problems (depending on #2)

As Ross points out -- the real problem isn't necessarily about the food spoiling -- it's about how much work your fridge has to do (and thus, how high the electrical bill is because of it). If you keep a mostly empty fridge, consider keeping bottles of water in the fridge to take up some space that never (or rarely) gets used. You can also do this with the freezer (and move them to the fridge if you need to reclaim the space).