Im sorry I don’t have the answer as how much panting is too much but I understand your worries! We’re in Texas too and our girls are about 12 weeks. I’ve started freezing bottles of water to put in their water. From what I’m understanding (I’m a first time chicken owner) that panting is how they cool down and as long as your providing them with cool water, shade and y’all put the fan out- they should be okay. Mine pant a lot but get playful when I come out and will eat & drink. I had to bring one inside today because she wouldn’t get up to eat or anything and was almost lifeless

. Hope the misters help, that’ll be the next thing I try.