24/7
skimmers are desiged to be run 24/7. If you shut them off, you impact the slime coating by allowing it to dry out which impacts the skimmer performance. Personally, i like a consistent skimmer and i also like the dissolved o2 that the skimmer provides. Not having it running reduces the dissolved o2 in the system when it's not running and it also impacts the skimmers overall performance.
There are many who run their skimmers part time but it's a practice i don't agree with for the reasons i mentioned above. There is really no upside in running a skimmer part time. Some might argue that it removes food during feeding but i'd argue that they need to find a better way to feed or slow the flow down in their tank so their fish can get the food before it goes over the overflow.