An excellent thread on Audit Scheduling is https://elsmar.com/ubb/Forum13/HTML/000159.html
Again, one should note that scheduling of audits is dependent upon your specific company and the type of audit. Those working for registrars are more consistent because of guidelines set up within Guide 62 and related documents.
The need to perform an audit, as well as frequency, is determined by the client.
Determining frequency should take into account:
Results of previous audits.
Status & Importance of the Activity.
Specified or regulatory requirements.
Significant changes in management, organization, policy, techniques or technologies.
Changes to the system itself.
Internal audits may be organized on a regular basis for management or business purposes.
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions