When pushing Nix store paths to Cachix, they are first checked for existance in upstream NixOS cache (to avoid wasting storage) and if not, they are compressed and uploaded.
You can switch between different storage plans to avoid reaching the storage limits.
Once you reach 85% of your storage limit, you will receive a warning email and another email when you reach the limit.
You can always check Garbage Collection page to see which store paths would be deleted first in case the limit is reached.
The following statistics are collected for each binary cache:
creation date of a pushed narinfo
last accessed date of narinfo and nar archive files
Garbage collection algorithm sorts all store paths by their last accessed date or creation date if the path was never accessed. It deletes the oldest entries up until your storage limit.
Due to a known Nix bug, Cachix will never serve a narinfo that does not contain full closure (all dependencies).