I know it might be hard to answer, since it might be difficult to tell time during a projection, but for those who have successfully projected, how long did it last?
They can last from 1 second to very long. My longest one felt like approx 15 minutes but I know they can last much longer. :-)
Some people say they can last all night and some say they have to periodically come back to their body. The longest I've managed is about two minutes I think.
it depends on the amount of experience for some reason. The longer you stay out, the greater the chance that you'll become unconscious and slip into a slight dream. Like Jarrod said, it helps to periodically come back to the body and project back out. Doing so probably refreshes your energy levels. As for going all night there would be only bits and pieces remembered. In general people usually slip unconscious after about 20 minutes, but it depends upon the individual.
As Timandra said, they can be anywhere from 1 second to ... anything really.
I think that there are too many factors involved to actually say anything beyond that.