[Phillip Speer]

Actually the pilot is responsible for his flight profile including the time to request descent. ATC may give you clearance to descend when ready to a certain level or to a particular level by a particular point, but they won't necessarily know when you need to descend. Remember though not to descend without ATC clearance to do so.
Randy's tip above at 3x1000ft (plus a bit to slow down as well) is a good one and qorks pretty much every time.

Phillip