As a retired ATCO, I am not aware of any regulation governing this situation. If the pilot has any doubt about this, all he has to do is ask. Otherwise, if I had instructed an aircraft to descend earlier than usual, I would expect the pilot to do so at a rate to suit himself; or for me to stipulate a required rate of descent - especially if this were for separation purposes. Actually, I cannot envisage any reason for ATC to issue such an instruction, other than for separation, or to comply with letters of agreement with other units. If this is critical, ATC would instruct a descent rate.