coral out of water

I have a frogspawn pretty high up on my rock work, when I do a water change about 2-3 heads are out of the water. How long could they be exposed to air before any ill effects happen? Should I move them a little lower? It really hasn't bothered me till now because it is getting so big and I don't want to do permanent damage to the coral. you can clearly see where my water change level is because from there up I have no coralline growth, so I am consistent with how much water is taken from the display tank. Thanks in advance.