I would guess your fish population matters the most for this question. If your fish are heat tolerant varieties, I would guess running 24/7 might be the more beneficial strategy. If you have less heat tolerant fish, then limiting the aeration during the hottest periods is likely the way to go. I personally have YP, SBS, and RES in my pond with seasonal tilapia. Bill tells me that the younger perch are more tolerant of heat, but as they grow larger (think ~14+") they become less tolerant of the heat, to the point that he has found some fish losses during those hottest days (and he's 3-4 hours north of me). To help combat this, he worked out that with my diffuser set up that I should achieve around 1 turnover for an hour of run time, therefore I run my diffuser for 1 hour over night. This allows my surface water to be as cool as possible before mixing, ideally allowing a little bit cooler escape during those dog days of summer.

Depending on your plant life and such, I've also been told you typically don't need much aeration up to around 6-7' as enough light should penetrate that deep to sustain plant life that will provide oxygen. I would guess pond dyes, turbidity, etc. would change that depth range.

[Linked Image from lh6.ggpht.com]