OCC Requirements-Reg 9.9 states-At least once during each calendar year, a national bank shall arrange for a suitable audit (by internal or external auditors) of all significant fiduciary activities, under the direction of its fiduciary audit committee, unless the bank adopts a continuous audit system in accordance with teh paragraph b of this section.
Federal Reserve - Industry Audit Guide - Fed recognizes continuous audit as well as one shot annual audit -Fed SR 83-17 states that exceptions to an annual audit should be documented
OTS - 12CFR 550.7 at least once during each calendar year an association's trust department shall be audited (either internally or externally)
The size of the trust department, complexity of business, degree of internal controls, organizational structure, and previously identified problems will affect the frequency and the level of testing.