I am really just a beginner with aeration, but love to "talk" it up, so take my ramblings with a tablespoon of sugar.
There are soooooo many variables and differences in ponds it is hard to give a "one size fits all" recommendation. Yet there is enough experience that at least we do have some general rules of thumb.
snrub's hit it right on the head with the above quote.
In very basic terms...
The fewer times you start the electric motor the cheaper it is to operate given that the overall runtime is the same. Starting and stopping a motor repeatedly during an hour requires a more energy than just letting it run all a hour long. This cost savings would be minimal with these little motors over the course of 12 hours and only 4 start/stop cycles. So, this savings could easily be ignored or next to impossible to realize on an electricity bill. (Backing up snrub again)
The fewer times you start the electric motor the less wear it will have given normal operating conditions (e.g. no excessive heat or moisture in the pumps environment).
With that said, let's consider running the aerator 12 straight hours, but realizing that the benefits are minimal compared to cycling on/off periodically throughout the day. Although, I am having a hard time arguing one way or the other, it could be chalked up to personal preference unless water temps come into play...see below.
With your pond in a northern climate, but not too far north, it may not make a lot of difference what time of day you run the system during the summer unless you have cold water fish like trout. At any rate, determine what water temps your fish like and run the aerators to accommodate that temp. e.g. if you think your water temp is too hot, then run the system only at night OR if its too chilly - run it during the day to utilize the day's heat to warm the waters. I expect my system to flip from nights to days as fall approaches and eventually be shut down during the winter.
I am finding that my aeration system (Gast 0523 and 3 vertex single diffusers) running from 9pm to 9am stabilizes the temps in general, but is not a real game changer unless the pond finds itself on the verge of a fish-kill. So far so good, I think of my aeration system as insurance and as a capacity increaser. It does cycle the muddiness of the pond though. Muddier in the morning and less so in the evening. I think cycling the system 4 times a day would keep the clarity, or lack thereof, more consistently muddier. I'm not say that is good or bad...just saying.
Here is a recent thread that I ramble even more if you care see some temp data from before and after aeration start-up along with some more of my theories...
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=495076#Post495076
