I understand that Work Units are dispursed based upon machine speed, so that WUs get completed in a week or so. This keeps WUs fresh from a scientific POV.
I have an AMD 3000 with a fast NVidia video card. Most of my units are about 3500 steps and take a couple of days. My present WU is 250 steps and will be finished in just over a day.
One's first WU is normally a bit smaller than normal.
I've put this on my CAD station. The WU I have is 500 steps.