You should leave your skimmer on all the time, especially if you have a reef or any inverts. A few (very few) salties out there say it's o.k. to run your skimmer 12 hours a day or every other day; totally false. Since fish produce waste 24 hours a day, you should run your skimmer 24 hours a day. This way, your water quality will not rise and fall. Changes in H20 will cause your fish to stress and this opens the door to disease and sickness. As for the second part of your question, the best way I have found to quiet a skimmer down, other than turning it off, is to put a canopy on...that is if your skimmer is a hang on the tank (H.O.T.) You can easily build one for about 1/2 the cost. It cost me about $80.00 for my 125 and $40.00 for my 55. Granted, I already had all of the hardware (hinges, screws, "L" brackets, etc, but that's only a couple of bucks worth of material. Go to your lfs and check out the designs of their canopies and/or go on-line and type in "diy canopies." Diy means "do it yourself." Hope this helps.
