I hear that some agencies are sticklers that it means 12 months, and some others that will allow 12-18, if all is well.
For sure...anyone who goes 18 months between independent audits had better have a really clean program.
We like to do our independent review every 12 months or so. We try to time it so it falls about 6 months after the regulatory audit. That way, if anything were found by either the regulators or independent auditors, we have time to address it before the next audit. It seems to work pretty well for us.