Our group used the 90 day rule as a limit for carriage of passengers but also as group currency.
Unfortunately one of the members who flew once a month had a few little incidents.He would take his children flying on the 89th day, but his airmanship was(is) cr*p.
In the end after he went 'off-piste' whilst taxying and trashed a prop , so we decided that we'd move to a 42-day currency. He didn't like that and left.
In the weather we've had over the last couple of years it seems a very reasonable requirement.