mel frank in several of his books talks about testing samples, and he found that at points later in flowering samples taken after 8 hours of dark tested up to 10% higher in THC. Notice the up to, and how it depended on when in flowering (I know it's in the insiders guide, one of his books). I've suspected that longer was just the result of people thinking more is better, but I dunno. I personally end up giving my plants anywhere from 20-30 hours of dark before I chop depending on my schedule at the time.