For me, it was not monthly, but rather “when it bites you in the ass”.

• Delete all empty rooms and federated rooms without local users.
• Clean up state_groups_state table.
• Delete old media.

All of this is made worse by having more active users and made better by having a large hard drive (my VPS had 20GB, which I almost filled up with the db and media after a few years, with only few users)