occurences
I don't think there is a certain statistic to say. it will depend on what your cham ingests. Soil, bark, rocks, high chitin foods, etc.. Straining to defacate. Straining to give birth, will all play a role as to if you experience one. Out of 15 chams, i have only had one. That happenend the day after i brought a rescue home. So it is quite possible you may never have one. If you do, a sugar water bath and some KY jelly might just do the trick. My chams went right away with one use of the KY jelly. He had eaten some dirt from his previous cage.
If you keep a plain cage floor and monitor their food, you probably won't have to deal with it. However, more serious ones can require a trip to the vet.