Microsoft’s Jason Langridge has a brief blog item about the support life cycle for Windows Mobile 2003 devices.
Windows Mobile Support Lifecycle
He says that official mainstream support for Windows Mobile 2003 ends in mid-2009. Of course, as Jason points out in his blog, everything really depends on the hardware manufacturer. Case in point: HP released the popular iPaq 2215 in 2003. It was a feature rich device in a nice small form factor. However, HP never provided an upgrade path from Windows Mobile 2003 to 2003 Second Edition (let alone Windows Mobile 5).
According to the software and drivers downloads page for the iPaq 2210/2215, there were three patches/utilities released for it in 2005 and nothing in 2006. So, it is pretty safe to say that this particular model had about 2.5 years of firmware support (mid-2003 to end-of-2005).