Mouse hover time = the delay time, in milliseconds, that the mouse pointer has to stay hovered on a item before tooltip popups display. If the hover time is set to 1000 milliseconds (just as an example) you'd have to hover for 1 second before the tooltip appears.
I believe (and I could be wrong) that the Extended UI Hover Time does not appear in the registry natively. I think it's a user created DWord value that helps extend or shorten the preview times in Aero Peek Preview, Live Preview (how long it takes taskbar thumbnails to appear), and possibly the sidebar gadgets.