Group expiration policy - what criteria does the policy use to determine when a group expires?

We are considering enabling group expiration on 80k+ groups.  Before we do so we would like to understand which groups will be affected.  There isn't any documentation that I could find on which criteria the group expiration policy uses, and when we opened a case with Microsoft they weren't able to tell us much other than 'check audit logs', which doesn't help since that only keeps data for 90 days. 

Does anyone have any insight on this one?  Maybe a way to run a 'what-if' scenario before we kick off the policy?  Thanks!