There is no single universal thickness applicable to every tooth and restoration. The clinical objective is to preserve sound marginal ridge and pericervical tooth structure wherever possible because loss of marginal ridges reduces cuspal stiffness and may increase fracture susceptibility. Preparation design should be determined by lesion extent, restorative material, occlusal loading, remaining wall thickness and pulpal or endodontic status.
Review the biomechanical and restorative considerations: dentalreach.today/are-marginal-ridges-marginalised-in-your-restorations
How do you modify preparation design when a marginal ridge is undermined but not visibly fractured?