Title says is all. Provide time last updated in update manager, dropdown dock icon, synaptic. and apt-get and aptitude should be more verbose with this too
ex:
$ apt-get status last updated on hh:MM:ss (we recommend you update)
update-manager:
Your system is up-to-date (as of hh:mm:ss)
You could expand the usability with a popup asking since you've updated already in the last hh:mm:ss you might want to reconsider until later. this can be implemented into apt-get aptitude update-manager synaptic.