After having done some cleaning, the storage quota can be recalculated on the page Settings -\> Usage Quotas, button "Recalculate repository usage" and wait several minutes to get a refreshed page up-to-date (the result is not instantaneous).
A grace period of 2 weeks preserves deleted objects. This can be shortened: https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html#space-not-being-freed
A grace period of 2 weeks preserves deleted objects. This can be shortened: https://docs.gitlab.com/ee/administration/housekeeping.html#prune-unreachable-objects
Unreferenced LFS files are garbage-collected once a day :
