Just what you want to hear, huh? It can take anywhere from a few seconds to a few hours depending on a number of different variables. Personally, I would say if after 2 hours, there's still no indication that it's moved at all, it's time to pull the plug on the testing.
I am running a diagnostics test on a Dell 4100 Dimension desktop (CTRL-ALT-D during setup). This is the first time I've done this. Right now, the screen is black and has the following text:
"IDE Drive Diagnostics running, please wait...
How long does it take to completet this test (a matter of minutes, an hour or so, etc.)? If nothing else happens after X minutes, is there a problem?