If you request descent, it means you want it now. Here, the controller would expect you to commence descent as soon as possible but not more than one minute. The same if the controller initiates the descent. If the controller has no restrictions and gets in first, the instruction may be preceded by "when ready descend to..." In that case, you can commence descent whenever it suits you or the computer. You could prefix your request with " request descent at (time/distance/place) or if planning ahead the controller may request your descent point.