Yes, 3000 is a good wait time. The first time you use the rubber ducky on a computer it takes a bit to install the drivers (could take a mintue or two, so plug in the ducky for the first time and wait. then reactivate the payload). But after the drivers install, 3000 is a standard wait time so that your computer recognizes that a new device has been plugged into your machine. If the script activates any sooner, the computer will most likely skip the beginning of the script since the payload activated before the computer recognized it's existance.
Of course, This all depends on the computer speeds. Very fast computers that have been duckied at least once, could get away with shorter wait times. But if you want to make your paylaod more globally accepted by computers, you haveto compensate for slower ones and add the delays at the beginning.