12 CFR §150.440

1. (a)Annual audit. If you do not use a continuous audit system described in paragraph (b) of this section, then you must arrange for a suitable audit of all significant fiduciary activities at least once during each calendar year.
2. (b)Continuous audit. Instead of an annual audit, you may adopt a continuous audit system. Under a continuous audit system, you must arrange for a discrete audit of each significant fiduciary activity (i.e., on an activity-by-activity basis) at an interval commensurate with the nature and risk of that activity. Some fiduciary activities may receive audits at intervals greater or less than one year, as appropriate.