Depends on what the situation is and why you want aeration.
I see several results from aeration-depending on the type- several things can occur. You can raise the water temp, you can lower the temp, you can add some O2 and you can limit anoxic water from forming.
Your highest BOD will likely be in late summer-early fall with plants starting to decompose, wild planktonic swings, high fish biomass and somewhat warm temps yet.
Choosing 3-5-4hrs of run time will be determined by the amount of what you have that you are trying to affect the most with aeration. Most times of the year you can do more damage by running too long of time period, but as I said, it depends why you need it.
YP vs LMB or BG.
YP handle pretty low DO but suffer greatly in higher temps-what would be comfortable for sunfish.
Having a shallow hypolimnion can benefit species like WAE, YP, etc. so with that said, if I had to choose 3.5hr to run it would be the 3.5hrs prior to sunrise.
There are some benefits to running mid-day at certain times but it's short. I think the best bang for the buck, buy batteries.
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