At the moment every 6 months there will be a new version of Ubuntu,
for instance the next is 'Intrepid Ibex' 8.10 (due 2008, October).

But if you go into your 'system - administration - software
resources', then click on the 'update' tab, you may see that you are
only set up to receive upgrade information on LTS updates. So in
theory you can go the next 3 years (or whenever to the next LTS)
without being told you can upgrade, thus you will get software updates
but skip over OS upgrades.

I only have hardy running on one of my systems at the moment (a 3 year
old laptop), will not run on my 6 month old machine due to an ACPI
issue, but I believe the next build of Hardy is due July, hopefully
that will clean up all the little bugs that have popped up.