It depends on the species, honestly. There are some that are completely exposed and in the sun for extended periods of time during low tide, including some Acropora. While there are a few that don’t need to be exposed at all, I’d say that most are fine for a few minutes. After all, when the divers in these remote islands go catch fish or harvest coral, they literally have colonies sitting in the floor of their boat or on the beach for a LOT longer than you would approve of.