I keep it in my sump. I keep it lit 24/7. I trim once a week with water change, and my tangs enjoy the clippings as a treat. I've never had it go sexual. I have read that even with 12-18 hours of light a day, they will be fine, as long as the time doesn't decrease gradually (signaling a change in seasons). And like seapug said, not harvesting it regularly will cause you problems as well. As long as you keep it on a consistent light cycle and harvest so it doesn't get too big, you will most likely be ok. I also read a post by someone who said that they have had it go sexual on them multiple times and it never killed any livestock.
Oh, and mine grows like weeds in the spring.