there is no set rule of thumb other than center leader trunk is cut off at no more than 24 inches. then side branches each is allowed to grow no more than 12 inches. I prune back. Then branches from them grow another 12 inches. I let them grow 24 inches so when I prune back to 12 inches, they are thick. Idea is to keep center of the tree open for light and air like a funnel. So cut off any growth that grows inwards towards the center.
My Nata was growing weird, so was pierce. I fixed nata and this year I am fixing pierce.
If you dont cut the center leader trunk off at low height, you charimoya tree will look tall and spindly. Folks worry about cutting/pruning, trust me these monsters grow fast in CA and I almost do two prunings.
Cherimoya trees should look like a bush and not a tree. Dr. Angel is who I got inspired by who maintains small height and his cherimoyas put large fruits too.