I had large fish, that had to be fed three times a day by same food as sun coral eats. Quarter of teaspoon each tine, plus food for other fish. Sun corals opened each time to pick up own share, within 15 min after food was added in water column. But they closed in ~1hr after daytime feeding.
In separate tank, sun corals only, fed once a day - they stay closed until 7:30 PM (19:30). If I have to feed them earlier, than usual, I add small amount of food in water with flow still on. Within 20 min they open to feed, and if this is evening - they stay open for hours.
I would say, feed it when you would like to see it open.
Or find a kind of dendrophyllia, that looks like sun coral, but is open all day long. Firecracker, if I remember right. Never had it, though, information is from dendrophyllia threads.