It varies by instance. PieFed has "spam detection" (not really) which is just a list of the posts and comments that have been downvoted a lot in the last 3 days so it's easy for admins to spot unreported badness and take action. I don't know how often other admins do but I check that report multiple times per day.

For example I nuked unlogopoco from piefed.social a month ago

Looking at the modlog for stermy, I'm seeing a fair bit of moderation action being taken on them and very few posts since then:

On piefed.social I banned townflex.com entirely weeks ago, which must be why I'm not seeing their latest spams.

Basically Lemmy has really poor tools for this which makes mods and admins do a lot more work then necessary, causing them to burn out. If they used PieFed they'd have a much better time.