Light Duration
 

It appears most people light their reef aquariums for 8-10 hours. It would seem there would be a benefit for coral growth if the duration were extended to 15 hours or perhaps longer. Is there a downside to longer photo periods beyond increase energy costs and maybe increased algal growth?

Most "experts" state 12 hours is what you should shoot for. There is no added benefit of any more. I don't remember exactly why.

The closer you get to the equator the light vs night becomes 12/12.
Since the corals are tropical it makes sense that they would never get an extended photo period in nature. They also wouldn't get intense light during that whole time either so 8 hours of strong MH makes sense to me. I run my 250w's for about that length of time and the rest of the photo period is with Fiji purpleT5's and 150w MH so nothing too intense.

also depends on wattage over your tank, less watts more time, more watts less time imho