In my experience it is highly individual.
I have two 3" (now likely 3.5) Philippine regals that ate from day one.
A smaller 1.25" never ate and died within a week.
A 4.75" Red sea that so far only goes on my zoas.
In the past I had a pair of blue bellies, likely from the Philippines. The smaller one took quite a while to start eating. When I later added the larger one and he saw the smaller one eating he joined right in.
I had one yellow belly from the Maldives that started eating straight out of the bag.
There is hardly much consistency, only that smaller individuals (2" to 4") are usually easier to take on frozen food, likely because they haven't specialized yet.
Another thing that may play a role is if these fish were in a pair before or still vagabonding juveniles/young adults. Adult regals form true bonded pairs.
I somewhat suspect that those that were bonded are somewhat depressed when separated and alone. In general I fond them doing better as a pair.
The prettiest are the ones from the Red Sea, closely followed by the ones from the Maldives. That said, the blue bellies look good too under the right light (blue)