On average it's far less than once a year that a new line is released.
For a little Dell Axim history:
Model...Release Date.....Time lapse:
X5.........Nov. 2002............-------------
X3.........Oct. 2003..........11 months
X30........May 2004..........7 months
X50........Sep. 2004..........4 months
X51........Sep. 2005..........12 months
However, Dell has decided to gracefully exit the PDA market. It's a result of a declining profit margin for PDAs, the drastic increase in smartphone sales (an area Dell has no plans to enter), and the fact that the X51 line already has the latest in technology, be it Windows Mobile 5.0, the VGA screen, or the 624MHz Xscale processor.
In short, Dell will stop manufacturing/selling Axims in the Spring/Summer of 2007. They will continue to offer product updates and support throughout the warranty period and will sell accessories such as batteries through 2009. Aside from that they do have plans to continue PDA development but they will be working at advancing the technologies of PDA lines manufactured by other companies, which will not be Dell/Axim branded.
What does that mean to you? For the time being absolutely nothing! The X51v is still the most powerful PDA available (and unless there's a major advance in technology will be for some time) and there will be no drop in support, so it's still the best option, for the time being and the long run. The only change is that this time next year you won't have the option of buying an Axim, leaving you with the main options of an overpriced HP iPaq or switching over to a Palm.
Hope this helps,
John