The whole logic board should be discharged in a few minutes top. But you never know for sure.

When you plug it again, look if the USB keyboard is receiving power. The light on the "Block Caps" key on Apple USB Keyboard turns on for a second when the computer starts up. I think that's why these PowerMacs often miss keyboard combinations at start.

Also try turning on the computer with no RAM modules installed. It should beep four times.