AS 3958.1 Ceramic tiles, Part 1: Guide to the installation of ceramic tiles stipulates a minimum screed thicknesses of 15 mm for bonded screeds and 40 mm for unbonded screeds. However, AS 4654.2 Waterproofing membranes for external above-ground use, Part 2: Design and installation states that an unbonded screed should be of a minimum thickness of 50 mm with lightweight mesh reinforcing.
By comparison, BS 8204 stipulates 25 mm for bonded screeds and 50 mm for unbonded screeds.
These thicknesses should be used as guide only. Many factors are required to be considered in order to determine the most appropriate screed/tile bed thickness. Each project is going to demand a unique screed thickness to ensure suitability, performance and durability based on the application, function and service conditions of the screed.