I will give you the source - this is not an excuse, just an explanation. Years ago I had a face-off with a 16949 CB auditor because some of our processes were on a 5-year internal audit cycle. The key words he pointed to were in the clause (in the wording that existed at the time):
9.2.1 The organization shall conduct internal audits at planned intervals to provide information on whether the quality management system: a) conforms to: 1) the organization’s own requirements for its quality management system; 2) the requirements of this International Standard; b) is effectively implemented and maintained.
Emphasis mine. His point was, during this registration cycle, how could we show the QMS conformed to requirements if we had not audited all of it? He at first said he was going to give us a nonconformity, but after I pushed for the shall it became an OFI in the end. We went ahead and changed the 5-year processes to 3-year frequency.
Anyway, there it is. Not a firm requirement, but an expectation that my CB also pushes but to my knowledge does not issue NCs for.