I only run one when I've done maintenance or a water change and have stirred up debris. If I know the tank is going to get dirty due to maintenance, I take the time to hose off the rocks with a turkey baster and stir up any debris, then I put a sock on and let it go until it's clogged (sometimes just a few hours), and take it off.
I get the desire to have "clear water" by running them all the time, but ultra clear water is not a good reflection of a natural coral reef environment - and at any rate, I've never had a problem keeping water clear (enough) even without socks.