When is the descent clearance requested from ATC?
Whether there are any formal policies for when to request descent will depend on the airline. However, I doubt formal procedures are established, since it should be pretty obvious to pilots when to request descent.

From a controller point of view, I will expect you to request descent when you are ready for it - so a minute or so before reaching your top of descent point. Controllers are able to estimate when you are going to request descent, based on experience, known level restrictions and typical aircraft performance. Controllers who have been working the same sector for a few years will be able to tell pretty much exactly when a certain flight is going to request descend, based on the current wind, runway in use, airline and aircraft type. What this means to you, as a pilot, is that you can expect the controller to be ready for your request; You can normally expect to receive a clearance for descent immediately after requesting it - so there is no reason to request it several minutes in advance.

Very few pilots will request descent several minutes before ToD. If they do, I can - if traffic flow permits - use the phrase "When ready descent to FL160". You are then free to start your descent when passing the ToD. Honestly though, if you call me up and say "Request descent in 5 minutes", I will most likely just reply with "Roger, call me back when ready for descent". The traffic situation can change significantly in 5 minutes, so I don't want to issue a clearance that may not fit into the future traffic picture. In the end, the early request just costs time on the frequency, which can be a scarce resource in a busy sector.

Moral of the story: as a controller, I know my airspace very well, and I have a pretty good idea of when you are going to be ready for descent. Because of this, there is no reason to request descent before you are ready for it. If you do, it will not matter at best, and be slightly annoying at worst.

ATC can of course also descend you before your ToD for various reasons, but that is another discussion.