As long as they want to. :)
Some will last for a matter of a couple days, some will seem to last weeks. Usually when you have a cold that lasts for "weeks" or a month, it is because your immune system is low and you are more likely to pick up other virus' on top of what you may have had originally.
Just make sure you are getting lots of rest and fluids and wash your hands frequently.
Feel better soon!!
