Drupal 7 OG Group Unsubscribe after given time period for member
This is definitely a job for rules. It is also quite a complexed subject as you are going to need to utilise components so that you can schedule this to happen every day. I would do it like this:


Set up a views_bulk_operations view that looks up all users that have been members of groups for 90 days.
Create a rule component that loads the list of users, loops through the list, sends a notification and unsubscribes the user from the group.
Schedule the rule to do this every day.