It typically will open at 6:30 for continental breakfast and 7am for full breakfast, on sea days.
On days in port, it will typically fully open at least one hour before scheduled arrival. Sometimes the 30 minutes of continental breakfast gets rolled in, sometimes there is still that separate 30 minutes.
The demographic for who eats when varies by cruise...its not the same every cruise. But generally, yes, it will be a little quieter closer to opening, particularly on sea days...everyone is sleeping in. Port days it can be pretty busy at opening. On the final day...disembarking day, it will be jammed at opening.