First we have to receive the rating report from the Tournament Director.
Depending on when and how the TD submits the rating report, it can take as little as an hour to rate an event or several weeks. As of September 2012, about 95% of the events we rate are now being submitted online, most of those are being submitted within 2 days of when the event ends and are rated within a few hours of when they are submitted online.
Events sent to the US Chess office by mail take longer, in part because of the time it takes for the event to get to the office. The typical event received in the mail is rated about 10-14 days after it ended. However, not all TDs get their events submitted promptly, sometimes we don’t receive an event until several months after it ends.