At least once annually.  All ISO Certification Audits follow a 3 year cycle including:

• 1st year is a full Stage 1 and Stage 2 certification audit
• 2nd year a Survelliance Audit (partial audit of the system)
• 3rd year is a  year is a Survelliance Audit (partial audit of the system)
• 3 Year cycle starts over on Year #4