Though each manufacturer marches to the beat of its own drummer, the bulk of the new camera announcements generally occur in January and February, in conjunction with the Consumer Electronics Association's and the Photo Marketing Association's annual trade shows, and in August and September, to gear up for the holiday shopping season.
Consumer snapshot cameras are usually updated every 12 months. For the most part, the updates take the form of a bump in sensor resolution, a modest performance increase, and the addition of support for the latest standards--this year's was PictBridge. More powerful cameras geared toward enthusiasts, such as digital SLRs, tend to have a slightly longer life cycle--anywhere from 15 months and beyond.
Of course, none of this helps to determine the best time to buy. That's because, until the digital camera market matures a little more, we expect cameras to continue to improve significantly with each generation. So for the near future, you can still expect to consider your model the hottest for only about a month.