It's actually on a 30 day basis and is done by user group promotions.
To do this, I use this add-on
https://xenforo.com/community/resources/user-criteria-by-waindigo.813/
You could also use this one instead
https://xenforo.com/community/resources/cta-criteria.3006/
I'm using the criteria "user has posted at least X times in the last Y days." I've set Y to 30 days and X to a minimum number of posts (I've not told users what this is). If they meet the minimum then they are promoted and don't see ads. They have to maintain the minimum as it's a rolling 30 day period, if they don't they are demoted and see ads until they have made enough posts again.