This may be spliting hairs but there is a difference IMO. Does an independent audit need to be performed every 12 months or is annually sufficient meaning it could be 18 months in between BSA audits?

What you already know from the examination procedures:

Independent testing (audit) should be conducted by the internal audit department, outside auditors, consultants, or other qualified independent parties. While the frequency of audit is not specifically defined in any statute, a sound practice is for the bank to conduct independent testing at least annually.

Interpreting it to mean once every calendar year would give you an even larger window... In the current environment, I would interpret it to be once every 12 months.

Give or take a month or two