FTR and in case anybody arrives here via search, here’s a working squid configuration that effectively caches packages forever (2 years) and meta information slightly less aggressively (four weeks):
# match against download urls for specific packages - their content never changes for the same url, so we cache aggressively
refresh_pattern -i (quarterly|latest)\/All\/.*(\.txz) 1440 100% 1051200 ignore-private ignore-must-revalidate override-expire ignore-no-cache
# match against meta-information - this shouldn't be cached quite so aggressively
refresh_pattern -i (quarterly|latest)\/.*(\.txz) 1440 100% 40080 ignore-private ignore-must-revalidate override-expire ignore-no-cache
refresh_pattern ^ftp: 1440 20% 1008000
refresh_pattern ^gopher: 1440 0% 1440
refresh_pattern -i (/cgi-bin/|\?) 0 0% 0
refresh_pattern . 0 20% 4320
maximum_object_size_in_memory 32 KB
maximum_object_size 2000 MB