In my opinion, the default is per the donor's stated payment schedule or preference (monthly, quarterly, semi-annually, or annually). Once that schedule has lapsed, the reminders should go out at least annually.
Annual giving pledges follow a different schedule as defined by that department. Those are all across the board but often begin with a 30-60-90 day frequency.
Electronic reminders are great! But should only be sent with a donor's consent.
John
John H. Taylor
Principal
John H. Taylor Consulting, LLC
2604 Sevier St.
Durham, NC 27705
919.816.5903 (cell/text)
Serving the Advancement Community Since 1987