How often should these inspections be required?

Neither the DL 100/2003, nor the Ordinance 369/2004, specify the frequency of inspections. Although a common reading of article 4 of this Ordinance seems to indicate that inspections should be annual, the article is not conclusive.

In the absence of any recommendation by the competent bodies, the entities responsible should always verify usage and the general state of the equipment and promote maintenance whenever necessary.  When in doubt of any technical and safety issue, the entity responsible should contact an inspection body to carry out new tests.