It's usually updated almost automatically. Someone (I assume the site publisher for each team) inputs that a given player is a commit and then the rankings change within minutes. This is how it normally works.
For some reason it's all screwed up right now as Maryland and Rutgers also pulled in commitments over the weekend that are not reflected in the national rankings, but do show up on their team sites.
This is not normal and I assume is a serious bug in the system right now. It usually updates in a couple of hours at most, assuming the player is rated, otherwise it takes a couple of dates sometimes to give him one.