The latest release of the D-Navi mapset was January 2005. I don't expect that they will stop releasing these updates. There is still a big market for D-Navi mapdisks (MB, VW, Audi, Ford etc). As long as you have a D-navi system, you should be in good shape; expect an update every 12-16 months. As long as you have the Jan 2004 or newer release, you should be in good shape.
However, If you have a C-Navi system (an older generation of the CD-based navi), then you might be looking at updates every 24+ months. I believe that NavTeq released an update in Feb 2004; and the previous C-Navi update was 2001 or 2002. See what I'm saying.
Be sure to see my posts on Map disks in the CD-based forum on this site.