How often do they cycle server unlocks?

Is there a set schedule to when they unlock/lock different servers to making new characters? Or is it just on a whim? Does anyone know?

It looks very much as though it's automatic based on current logged-in population, so you can often create on many worlds in the wee hours of the morning. That said, now that everyone's figured out there's no AFK timer, don't expect that to work anymore...