Check Airman's response appears to quote the Aeronautical Information Manual published by the US FAA.The 7110.65 (FAA controller's handbook) informs controllers that pilots are expected to commence descent "promptly" upon receipt of the instruction, so it's likely what ATC expects of all pilots when operating in US airspace.
But it is nice to know that should i ever visit Australia, I can take my time starting that descent!
