There's no specific legal requirement - the law (building regs) is phrased more in what needs to be achieved (maintain stability etc) rather than the details how to cut a chase. The means of achieving that will vary with the exact circumstances - e.g. type of construction and materials, but the usual rule of thumb (from memory) for solid brick/block walls is 1/3rd the thickness of the brick/block for vertical chases and 1/6th for horizontal ones (so you ignore any plaster/render) - specific circumstances may permit deeper chases though, or require shallower ones. For a cavity wall, you consider each leaf as a separate wall for these purposes.
