by Randy Carey: Every 90 days is more restrictive than that is required under Regulation D, so it would be a matter of your agreement with the customer.
by John Burnett: If your bank is willing to make itself less attractive, you can impose a six transfers per 90-day statement period, since it is more restrictive than the regulation requires.
What you SHOULD be doing is counting covered transactions per roughly one-month period. The period can't be shorter than four weeks, but can be as much longer than 4 weeks as you wish. You do not have to match the counting period to the statement period, although that may be the easiest way for you to show the customer how s/he "colors outside the lines."
The one thing you cannot do is to think that 6 per month means 18 per quarter. That would allow the customer to make 18 transfers in the first week of the quarter, and that's not permitted.