We see them most frequently in the mid to late morning, up til about noon.
I know from experience that they also hunt a lot in the late afternoon, but I although I see them frequently around our place in the morning, they must have something else to do in the afternoons, as I never see them around past lunch.
I would not count on the above as advice, though. They're a hazard anytime there's potential food out there for them.
btw - I saw the largest Cooper's Hawk I've ever seen dive-bombing my run the other day at about noon, but the run's roofed and has hardware cloth over the "open" parts of the run, so he went away hungry.
We leave the GSD out when the chickens are out of the run and foraging, he chases off everything from blue jays to 737s.