I would suggest managing these structures in the same manner as your bridges and major culverts. The inspection process for the VicRoads "Road Inspection Manual" applies to retaining walls, and they stipulate their inspection frequency.
A Link to the manual can be found here:https://www.vicroads.vic.gov.au/-/media/files/technical-documents-new/road-structures-inspection-manual/rsim-parts-1-and-2_oct_2018.ashx
Frequency can be determined based on risk I believe, however we try to maintain 6 monthly level 1's and 2 or 4 yearly level 2's based on the short (timber and steel) and long life (Concrete) structures.
Give me a call if you need to discuss
T: 1300 416 745