Neither of the ISO 9001, 17001, 19011 standards define the validity period of the certificates for internal auditors. There is an opinion that there is no need for such a certificate at all - no direct requirement - to conduct the internal audits. However that is why all the certificates I saw refer to the fact of completing a relevant training program.
Unless it is specified in the training program by the issuing body, the certificate issued has no expiry. Another source to consider is IATF 16949 that imposes requirements to maintain and improve the competence of internal auditors.
Then if your certificate is issued under the CQI/IRCA auspices, the CQI/IRCA auditor's validity is five years only. To maintain the status one has to do CPD (Continuous Professional Development) and audits.
The charted re-certification procedure can be found at www.quality.org . There is a requirement for minimum number of audits per year performed by auditors.
