Just wondering. I know it's dangerous to handle power supplies because even when unplugged they can hold power. How long does it have to be unplugged before the power is gone?

As with most things, it depends if the internal (series) resistance is high it will disipate quickly, if it is low it could be quite a while maybe even days before you would be safe. So when handling large caps you need to assume that they are charged and dangerous,. Only way to be sure is to short out the leads with a large resistor and that can still give a pretty good jolt. I was disassembling a digital camera and got a big shock, when I accidentally touched the leads to the Cap for the camera flash. I assumed that since I discharged the cap it wouldn't discharge again. Well obviously I got shocked again, after the third shock there wasn't much left but even the third shock on a cap about the size of a AA battery gave me three big jolts.