I only run my skimmer for about 3-4 hours a day............I used to run it 24/7 but had an issue with it overflowing sometimes so now I start it at about 5 am and it turns off right before I go to work.
I believe its actually benificial to my corals because I do my feedings at around 7-8pm and and the skimmer would suck up all the micro food that the corals feed on before they actually have a chance to eat it...........so by only running it a few hours it keeps the good stuff in the water longer but still has a chance to remove the bad stuff on a daily basis.............THis is just me opinion...........I could be completly wrong