They really can't tell you an exact time because it depends on so many variables such as the ship's arrival time and most importantly when Customs has released the ship. No one may leave the ship until Customs says so and they don't have to rush if they don't want to. Once Customs says 'okay, you may start debarkation', the self-assist people are one of the first groups which may leave if I recall correctly. Once a group is called that doesn't mean you have to leave at that time or else. You can leave at any time after your group is called. However, they like to get the self-assist people out of the way first because they would otherwise clog up the walkways with all of their luggage and this causes crowding, tripping hazards, etc.
What ship are you going on, and from which port will you be cruising? Someone here may be able to give you a ballpark time that debarkation usually starts. I'd say it rarely would be before 8am anywhere, so I'd just tell my group to ensure they had eaten and were ready to go by that time.