\"apt-get autoclean\" could be run regularly by cron, so that \"the cache will be maintained over a long period without it growing out of control\". A corresponding script could be installed by the apt package to be run daily/weekly/monthly... --- --- current release: apt-0.3.19cnc55-alt3
I think weekly should be fine for the start, please see apt-autoclean at nearby sisyphus mirror tomorrow.
apt-autoclean-0.1-alt1 -> sisyphus: * Sat Oct 09 2010 Michael Shigorin <mike@altlinux> 0.1-alt1 - created for ALT Linux (closes: #918) + based on makewhatis weekly job script