That's a question I have too. They haven't talked about that as far as I can discover, and I don't know enough about orbital mechanics as it relates to approaching the L2 point to be able to figure the answer out from first principles.
Perhaps they will reveal this in the coming days.
I started clicking on all the links at the Where Is Webb page and found some helpful answers.
The orbit around L2 will be approximately the same distance as the orbit of the Moon around the Earth.
However, one complete orbit around L2 will take about six months, compared to about 4 weeks for the Moon.
Relative to Earth, the L2 orbit is a polar orbit, not an equatorial orbit.
Also relative to Earth, the L2 polar orbit would never be on the dark side of the Earth.
Instead, the L2 orbit resembles an Earth polar orbit that is always facing the sun.
I made some quick calculations, and it looks like the Webb orbital speed will be around 325 mph.
The Moon orbits around 2,300 mph.