The minimum recommended axial depth for the gingival floor in a class II cavity preparation is 0.8 mm to ensure adequate removal of carious dentin while preserving tooth structure. This depth allows sufficient bulk of restorative material to provide mechanical strength and resistance to fracture. Less than 0.8 mm may compromise restoration durability, while excessive depth risks pulp exposure. Maintaining this dimension also helps in achieving proper contour and contact with adjacent teeth for optimal function and hygiene.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.