We automatically prune attachments (remote files stored by your instance, 'cached') about every 2 weeks.
Files older than 90 days will be considered for deletion, and it will show up as Tasks and free up media storage space.
Once every few months, extra jobs are done in this periodic task. They will clean up any forgotten attachment, and then try to compact the database by removing remote statuses that have not been interacted with.
You might have access to settings overrides in Mastodon's own settings, but the 90 days limit is enforced by our file storage provider and so going lower will not actually free space.