I haven't worked in Oz for a long time now, but from memory in the Australian MATS it says that any instruction to pilots must be followed within one minute of recieving the instruction. If that is the case then when an aircraft has asked for descent and descent has been given, at least in Oz, that aircraft must have started descent within one minute. It isn't unfortunately in the books here in the Middle East, so I am assuming it must just be an Oz only thing, but for me it's a great rule to have.
It happens here all the time and in our case I find Qatari and Gulf Air are the worst offenders of asking for descent and then maintaining for 4 or 5 minutes. You shouldn't have to say "descend now" in my opinion if the pilot has requested descent.