I would suggest 8 hours as average. Some do less to avoid algae problems, but corals need light, so I use 8 hours of intense illumination with an hour in the morning and evening of just stunner strips.
stunner strips just sound dangerous. i personally like to use a staggered photoperiod. my first set of whites come on at 10a, then my second set at 1030. then the first set shut down at 7p, followed by the second set at 730. they do actually flip now that i think about it because only one set of my whites activate the cooling fans. so those lights come on first and shut off last to try and help remove as much heat from the fixture as possible.