My zonals are in planters, when in hot heat and full sun, I water every second day. I find then the leaves start to wilt in the slightest, they need water immediately. However I'm in a much different zone than you, I'd consider hot here about 77F/25C.

In the ground they'll need less frequent watering than in containers. As to how often that should be it's an impossible question to answer. The only way to decide if a plant needs water is to observe it and respond accordingly.

I always have zonal geraniums in full sun. I live in zone 7 but that's a little misleading for summertime. It is normally 95 - 105 and sunny. They are in containers and I water them with a hose daily. I have a drip system for my flowerbeds and run that twice a week (the bed is just in front of the containers).