I would run it 24 hours a day 7 days a week. Aeration is the number 1 best thing you can do for your pond. If the pond is older you will need to gradually work up to 24 X 7 or you could cause a fish kill. Don't be scared when you first have a really bad smell coming up from the bottom. It will go away once the aeration has a chance to take effect.

We stop aeration in November and start again in April. Could super cool the water up here if we leave it on all winter.