Based on my experience in industry, internal audits are conducted on a regular basis (usually once a year) by someone who works for the company. This is just a requirement that most companies have in their procedures and makes sure that product DHFs are in conformance with the company procedures. It is a good way to ensure that there is consistency between different projects, that good documentation practices are being followed, and that any mistakes that are found are fixed. Sometimes companies may hire outside contractors to audit them, and this usually does not occur on a regular basis, but is just something that the company does to make sure they are prepared for an audit by FDA, ISO, or a notified body. FDA can come audit at any time, but I do not think that they audit on a regular basis. The important thing is that the company is always ready if FDA were to come knocking on the door unannounced.