I think the only way to do something would be to look at there name and view change history and see when they were moved into that usergroup I don't think there's any other way.
Sorry for the lack of knowledge about it but I'm really not sure there's anything like that unless you give them a month and move them out with user promotions make it so there's different users-groups for more lengthy subscriptions or something,
Hope you understand what I'm trying to say!
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