When does the Postage Rate Change?

Postal rates changed for a first class one ounce stamp on May 11, 2009. The cost rose from $0.42 to $0.44 for a one ounce letter. The post office has not announce another rate increase yet but since there has been an increase the last couple of years it is not unrealistic to expect one. The one item in the news has been decreasing the number of days of delivery rather than increasing the cost of stamps.